Metal enhanced fluorescence (MEF) for biosensors: General approaches and a review of recent developments

Templating assisted fabrication of flexible, highly stable and uniform plasmonic platform for ultrahigh enhancement of Raman and fluorescence signals: Model sensing of rhodamine-6G

Raman-fluorescence dual enhancement offers transformative potential in fields like biosensing and bioimaging, combining exceptional sensitivity with rapid response. In this study, a novel flexible plasmonic platform, composed of silver nanocolumns (Ag-NCOLs) embedded in polydimethylsiloxane (PDMS), was investigated using 4-aminothiophenol (4-ATP) for Raman and rhodamine-6G (R6G) for fluorescence studies. This platform demonstrated remarkable enhancement factors (EFs) of ∼1014 for Raman and ∼273 for fluorescence, surpassing Ag-NCOLs on silicon (Si) by factors of 10 and 24.8, respectively. A number of control experiments confirmed the platform's uniformity, long-term stability, and flexibility (bending and stretching). The system's efficacy was validated through R6G detection, achieving detection limits of 0.54 nM for Raman and 0.26 nM for fluorescence. These findings establish the platform as a promising tool for diverse applications, including label-free and labeled biosensing, pathogen detection, bioimaging, in vitro diagnostics, wearable sensors, material characterization, and environmental monitoring. This work underscores the immense potential of flexible plasmonic platforms for advancing next-generation sensing and imaging technologies.

In situ Metal-Enhanced Fluorescence of gold nanoclusters for enzymatic biosensors

A fluorescent enzymatic biosensor based on the in-situ observation of the MEF (Metal- Enhanced Fluorescence) effect has been developed for the first time. It has been demonstrated that during the enzymatic reaction of cadaverine with Diamine Oxidase (DAO) (used as proof of concept) in the presence of Au (III), the product can reduce the gold precursor to form AuNPs. When the reaction takes place in the presence of silica-coated gold nanoclusters (AuNCs@Si), an increase in the fluorescence signal is observed, which can also be related to the concentration of cadaverine. To understand the mechanism, the interaction between the AuNCs@Si and the enzymatic reaction, as well as their interaction with Au (III), has been studied. Under optimal conditions, the method shows a linear response range up to 250 μM Cadaverine, a limit of quantification of 52 μM and a limit of detection of 16 μM, which is about two orders of magnitude lower than the method without MEF (DAO/cadaverine/Au (III). It has been applied to the determination of cadaverine in spiked oily fish with a recovery of 95 ± 8 % (n = 3) and no significant differences (P = 0.05) were observed compared to a reference method. This MEF effect was also observed for the enzymatic determination of both xanthine with xanthine oxidase and tyramine with tyramine oxidase.

Plasmon-enhanced fluorescence sensor based on Au nanocages for sensitive detection of norepinephrine

BACKGROUND

Norepinephrine (NE) as a crucial monoamine neurotransmitter in the central and sympathetic nervous system, plays an important role in different physiological and pathophysiological processes. Brain NE can modulate cerebrospinal fluid flux and neurovascular coupling, regulate cortical and hippocampal neuronal circuitry, and participate the immune system. In addition, the reduced concentration of NE in brain was currently deemed to be the internal reason of major depression. However, development of detection method of NE with high spatiotemporal resolution in living systems remains a great challenge.

RESULTS

Herein, a plasmon-enhanced fluorescence (PEF) sensor based on Au nanocages (Aucages) were designed and prepared for ultra-sensitive detection of NE. Aucages with porous walls, hollow interior and systematically tunable localized surface plasmon resonance (LSPR) wavelengths (536 nm, 654 nm, 754 nm) were prepared to obtain a highly fluorescent enhancement of Au nanoclusters (Au NCs). Moreover, polyethylene glycol (PEG) with different molecular weight (1000, 5000, 10000 Da) were applied to control the distance between the Aucages and Au NCs. 3D-FDTD simulation results indicated that the fluorescence enhancement was primarily due to the internal and external enhanced electric field effects of Aucages. This sensor was applied for the turn-on detection of NE in commonly used clinical injectable norepinephrine bitartrate with the recovery rate of 98.06-105.34 %. Meanwhile, real-time fluorescence imaging of NE in living pheochromocytoma (PC-12) cells was explored with a red-emitted fluorescence.

SIGNIFICANCE

This study first employed Aucages with more "hot spot" for red-emitted Au NCs to realize fluorescence enhancement. It provides a new method for the development of more sensitive, accurate and convenient analysis of NE in clinical drug analysis, cell monitor and metabolism study.

Plasmon-enhanced fluorescence of polyvinylpyrrolidone-embedded CdSeS/ZnS quantum dots on gold film over nanospheres

This study explores the fluorescence enhancement of quantum dots (QDs) by gold film over nanospheres (AuFoN) plasmonic substrates, focusing on how a polymer matrix and plasmon resonances of the substrate affect the fluorescence properties of QDs. It was observed that polyvinylpyrrolidone (PVP) facilitated the uniform distribution of QDs on the surface of the AuFoN by simple drop-coating, avoiding aggregation during solvent evaporation. Progressive fluorescence redshifts and intensity enhancement were observed when moving from QDs on glass substrates to planar Au, and most pronouncedly, to nanostructured AuFoN substrates. The fluorescence enhancement was further analyzed by varying the diameter of the polystyrene spheres used in AuFoN fabrication, revealing that substrates based on 600-700 nm spheres provided the strongest fluorescence amplification due to stronger localized electromagnetic fields. Time-resolved fluorescence measurements revealed two primary fluorescence lifetime components for QDs on AuFoN: a short component linked to non-radiative plasmonic energy transfer and a long component representing intrinsic QDs emission. By optimizing sphere size, Au nanostructured films can be tailored to control QDs fluorescence lifetimes and intensity, advancing their use in biosensing, photonics, and other fluorescence-based technologies. This work enhances our understanding of how substrate design and matrix effects impact QDs fluorescence, providing a pathway for precisely engineered Surface Enhanced Fluorescence (SEF) platforms suited to various applications in optical sensing and more general photonics.

Piezoelectric-Driven Amplification of Plasmon-Enhanced Fluorescence for Advanced Sensing Applications

Fluorescence based detection is applied across various fields, including medical diagnostics and environmental sensing. A key challenge in these technologies lies in optimizing sensitivity through enhancement of the fluorescence signal. In this study, we demonstrate that combining piezoelectric and plasmonic processes increases the fluorescence yield. Piezoelectric poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP), is utilized as an external electric field modulator to produce a reliable and reproducible fluorescence enhancement of InP/ZnS quantum dots approaching the single nanoparticle level. The relationship between the applied force and the fluorescence response is both experimentally quantified and theoretically modeled and the dependence of the fluorescence enhancement on the excitation wavelength and on the PVDF-HFP substrate topography is elucidated. Furthermore, fluorescence enhancement by a magnitude of order for a DNA hybridization assay on the gold-coated PVDF-HFP substrate is demonstrated, highlighting the practical applicability of this approach in biosensing.

Integrative plasmonics: optical multi-effects and acousto-electric-thermal fusion for biosensing, energy conversion, and photonic circuits

Surface plasmons, a unique optical phenomenon arising at the interface between metals and dielectrics, have garnered significant interest across fields such as biochemistry, materials science, energy, optics, and nanotechnology. Recently, plasmonics is evolving from a focus on "classical plasmonics," which emphasizes fundamental effects and applications, to "integrative plasmonics," which explores the integration of plasmonics with multidisciplinary technologies. This review explores this evolution, summarizing key developments in this technological shift and offering a timely discussion on the fusion mechanisms, strategies, and applications. First, we examine the integration mechanisms of plasmons within the realm of optics, detailing how fundamental plasmonic effects give rise to optical multi-effects, such as plasmon-phonon coupling, nonlinear optical effects, electromagnetically induced transparency, chirality, nanocavity resonance, and waveguides. Next, we highlight strategies for integrating plasmons with technologies beyond optics, analyzing the processes and benefits of combining plasmonics with acoustics, electronics, and thermonics, including comprehensive plasmonic-electric-acousto-thermal integration. We then review cutting-edge applications in biochemistry (molecular diagnostics), energy (harvesting and catalysis), and informatics (photonic integrated circuits). These applications involve surface-enhanced Raman scattering (SERS), surface-enhanced infrared absorption (SEIRA), surface-enhanced fluorescence (SEF), chirality, nanotweezers, photoacoustic imaging, perovskite solar cells, photocatalysis, photothermal therapy, and triboelectric nanogenerators (TENGs). Finally, we conclude with a forward-looking perspective on the challenges and future of integrative plasmonics, considering advances in mechanisms (quantum effects, spintronics, and topology), materials (Dirac semimetals and hydrogels), technologies (machine learning, edge computing, in-sensor computing, and neuroengineering), and emerging applications (5G, 6G, virtual reality, and point-of-care testing).

Fluorescent silver hydrosol for the dual fluorometric sensing of gallic acid and Cd2

Herein, silver-enhanced fluorescence was obtained using a small molecule of salicylaldehyde (SL) in an alkaline solution in the presence of silver nitrate. Ag+ was reduced to Ag0, while SL was oxidized to the quinone form OSL. OSL was a very weak fluorophore and exhibited metal-enhanced fluorescence (MEF) in the presence of proximal silver nanoparticle surfaces. MEF was selectively quenched by gallic acid and restored by Cd2+. Thus, a dual-sensing platform was obtained for gallic acid [limit of detection (LOD) = 1.77 × 10-5 M; linear detection range = 10-8 to 5 × 10-5 M)] and Cd2+ (LOD = 2.4 × 10-6 M; linear detection range = 5 × 10-8 to 10-6 M) in a one-pot. The observed lightning rod effect and higher radiative decay rate were ascribed to MEF, while replacing the capping agents was responsible for the tuning of MEF. The sensing protocol was applied to natural samples from real sample sources, and satisfactory results were obtained.

Interfacial mechanisms of enhanced photoluminescence in AgI-doped red light emitting perovskite quantum dot glass

Red light emitting perovskite quantum dot (PQD) glass, with narrow-band emission and excellent stability, holds great potential for applications in liquid crystal displays. However, its low photoluminescence quantum yield (PLQY) remains the biggest obstacle limiting its practical application. Additionally, the mechanism behind the enhancement of the PLQY is not well understood, which restricts the further improvement of the PLQY in red light emitting PQD glass. In this work, AgI doped red light emitting CsPbBrI2 PQDs glass exhibitated significantly improved PLQY. The underlying mechanisms shown that the formation of Ag nanoparticles (NPs) promotes a localized surface plasmon resonance (LSPR) effect, which increases the charge carrier density and optical activity. Density functional theory (DFT) calculations indicate a widening of the PQD bandgap upon AgI doping, which contributes to reduced non-radiative recombination and improved PLQY. Attractively, the optimal doping concentration of 0.4 mol% AgI resulted in a maximum PLQY of 62.4 %. Finally, the PG0.4 sample demonstrated excellent thermal, water, and photostability, maintaining over 88 % of its initial emission intensity under prolonged stress conditions. These results highlight the potential of AgI-doped red light emitting CsPbBrI2 PQDs for use in high-performance optoelectronic devices, offering both enhanced luminescence and long-term stability.

PhotoChem Interplays: Lighting the Way for Drug Delivery and Diagnosis

Light, a non-invasive tool integrated with cutting-edge nanotechnologies, has driven transformative advancements in imaging-based diagnosis and drug delivery for cancer and bacterial treatments. This review discusses recent progress in these areas, beginning with emerging imaging technologies. Unlike traditional photosensors activated by visible light, alternative energy sources such as near-infrared (NIR) light, X-rays, and ultrasound have been extensively investigated to activate various photosensors, achieving high sensitivity, wavelength versatility, and spatial resolution for deep-tissue imaging. Moreover, to address challenges like tissue autofluorescence in real-time fluorescence imaging, afterglow luminescent nanoparticles are being developed by integrating these alternative energy sources for real-time imaging and sensing in deep tissue for precise cancer diagnosis and treatment beyond superficial tissues. In addition to deep tissue imaging, light-responsive nanomedicines are revolutionizing anticancer and antimicrobial phototherapy by enabling spatially and temporally controlled drug release. These smart nanoparticles are engineered to release therapeutic cargo at target sites in response to microenvironmental cues specific to tumors or infections. In anticancer phototherapy, these nanoparticles facilitate controlled drug release via photoisomerization, photothermal, and photodynamic processes. To enhance circulation time and specific targeting, biomimetic nanoparticles, which mimic natural anti-tumor responses by our body, have attracted increasing attention. In antimicrobial phototherapy, research has been focused on the chemical modification of the photosensitizer to enable targeted drug delivery. An intriguing strategy has recently emerged involving the development of "pro-photosensitizers" that are specifically activated within bacterial cells upon light irradiation, offering a high margin of safety. These advancements leverage photochemical reactions and nanotechnology to enhance precision therapy and diagnosis in addressing critical health challenges.

Smart Luminescent Materials for Emerging Sensors: Fundamentals and Advances

Smart luminescent materials have drawn a significant attention owing to their unique optical properties and versatility in sensor applications. These materials, encompassing a broad spectrum of organic, inorganic, and hybrid systems including quantum dots, organic dyes, and metal-organic frameworks (MOFs), offer tunable emission characteristics that can be engineered at the molecular or nanoscale level to respond to specific stimuli, such as temperature, pH, and chemical presence. This adaptability makes them crucial in developing advanced sensor technologies for environmental monitoring, biomedical diagnostics, and industrial applications with the help of the luminescence mechanisms, such as fluorescence, phosphorescence, and upconversion. Recent advancements have been driven by the integration of nanotechnology, which enhances the sensitivity and selectivity of luminescent materials in sensor platforms. The development of photoluminescent and electrochemiluminescent sensors, for instance, has enabled real-time detection and quantification of target analytes with high accuracy. Additionally, the incorporation of these materials into portable, user-friendly devices, such as smartphone-based sensors, broadens their applicability and accessibility. Despite their potential, challenges remain in optimizing the stability, efficiency, and biocompatibility of these materials under different conditions. This review provides a comprehensive overview of the fundamental principles of smart luminescent materials, discusses recent innovations in their use for sensor applications, and explores future directions aimed at overcoming current limitations and expanding their capabilities in meeting the growing demand for rapid and cost-effective sensing solutions.

Designing Fluorescent Interfaces at Hotspots in a Plasmonic Nanopore for Homologous Optoelectronic Sensing

In this work, a site-selective functionalization strategy is proposed for modifying fluorescent dyes in the plasmonic nanopore, which highlights building optoelectronic dual-signal sensing interfaces at "hotspots" locations to construct multiparameter detection nanosensor. Finite-difference time-domain (FDTD) simulations confirmed the high-intensity electromagnetic field due to plasmonic nanostructure. It is demonstrated that adjusting the distance between the nanopore inner wall and fluorophore prevented the fluorescence quenching, resulting in more than a thirty fold fluorescence enhancement. Upon binding with the target analyte, the sensor produces homologous yet independent optoelectronic dual-signal responses that cross-validate one another, providing highly accurate analysis even in the presence of multiple interferences. The platform demonstrates precise, adaptable detection with linear responses to extracellular pH changes at the single-cell level, making it a versatile tool for a range of biosensing applications. By enabling the functionalization of fluorescent interfaces in the "hotspots" of metal nanopores, this interface design strategy efficiently exploits the enhancement of electromagnetic fields to achieve high-precision dual-signal measurements and greatly improves the sensitivity of biosensing applications.

Hybrid Plasmonic Symmetry-Protected Bound state in the Continuum Entering the Zeptomolar Biodetection Range

Photonics bound states in the continuum (BICs) are peculiar localized states in the continuum of free-space waves, unaffected by far-field radiation loss. Although plasmonic nano-antennas squeeze the optical field to nanoscale volumes, engineering the emergence of quasi-BICs with plasmonic hotspots remains challenging. Here, the origin of symmetry-protected (SP) quasi-BICs in a 2D system of silver-filled dimers, quasi-embedded in a high-index dielectric waveguide, is investigated through the strong coupling between photonic and plasmonic modes. By tailoring the hybridizing plasmonic/photonic fractions, a trade-off is selected at which the quasi-BIC exhibits both high intrinsic Q-factor and strong near-field enhancement because of dimer-gap hotspot activation. Not only radiation loss is damped but in a configuration sustaining a lattice of plasmonic hotspots. This leads to an advantageous small modal volume for enhancing light-matter interaction. The layout of nearly embedded dimers is designed to maximize the spatial overlap between the optical field and the target molecules, enhancing reactive sensing efficiency. The architecture is evaluated for its ability to detect transactive response DNA-binding protein 43. The refractometric sensitivity outperforms current label-free biosensing platforms, reaching the zeptomolar range. The approach highlights the potential of combining plasmonic and dielectric nanomaterials for advanced sensing technologies.

Synthesis of SiO2@Ag Nanocomposite for Investigating Metal-Enhanced Fluorescence and Surface-Enhanced Raman Spectroscopy

SiO2@Ag nanocomposite (NC) has been synthesized by the chemical reduction and Stӧber method for Metal-enhanced fluorescence (MEF) of Rhodmine 6G (R6G) and Surface-enhanced Raman spectroscopy (SERS) of Malachite green (MG). As-synthesized SiO2@Ag NC indicated SiO2 nanosphere (NS) and Ag nanoparticle (NP) morphologies. The SiO2@Ag NC was high quality with a well-defined crystallite phase with average sizes of 24 nm and 132 nm for Ag NP and SiO2 NC, respectively. By using SiO2@Ag NC, the photoluminescence (PL) intensity of the R6G (at 59.17 ppm) was increased approximately 133 times. The SERS of the MG (at 1.0 ppm) with SiO2@Ag NC as substrate clearly observed vibrational modes in MG dye at 798, 916, 1172, 1394, and 1616 cm-1. As a result, the SERS enhancement factor (EFSERS) at 1172 cm-1 obtained 6.3 × 106. This initial study points to the potential of SiO2@Ag NC as a promising material for MEF and SERS substrates to detect dyes at low concentrations.

Color-Coded Traffic Signal Method Combined with Nanodiamond Quantum Sensing for Accurate miRNA Detection

Background noise interferes with the accurate detection of early tumor biomarkers. This study introduces a method that effectively reduces background noise to enhance detection accuracy by combining a color-coded signaling approach with the unique fluorescent properties and room-temperature tunable quantum spin characteristics of fluorescent diamonds (FNDs) with nitrogen-vacancy centers. In this approach, a red signal indicates the presence of the target analyte within the spectral region, a green signal indicates its absence, and a yellow signal indicates the need for further analysis using FNDs' quantum spin properties for optical detection magnetic resonance (ODMR) to distinguish the FND signal from background noise. Preliminary results demonstrate that this method enables the detection of breast cancer-related miRNA-21 and miRNA-96 concentrations as low as 1 fM within a 100 × 100 μm2 area, achieving single-molecule detection capability. This method is suitable for accurate biomarker identification and detection under high-background fluorescence conditions.

Innovations in one-step point-of-care testing within microfluidics and lateral flow assays for shaping the future of healthcare

Point-of-care testing (POCT) technology, using lateral flow assays and microfluidic systems, facilitates cost-effective diagnosis, timely treatment, ongoing monitoring, and prevention of life-threatening outcomes. Aside from significant advancements demonstrated in academic research, implementation in real-world applications remains frustratingly limited. The divergence between academic developments and practical utility is often due to factors such as operational complexity, low sensitivity and the need for trained personnel. Taking this into consideration, our objective is to present a critical and objective overview of the latest advancements in fully integrated one-step POCT assays for home-testing which would be commercially viable. In particular, aspects of signal amplification, assay design modification, and sample preparation are critically evaluated and their features and medical applications along with future perspective and challenges with respect to minimal user intervention are summarized. Associated with and very important for the one-step POCT realization are also readout devices and fabrication processes. Critical analysis of available and useful technologies are presented in the SI section.

Plasmonic nanoparticle sensors: current progress, challenges, and future prospects

Plasmonic nanoparticles (NPs) have played a significant role in the evolution of modern nanoscience and nanotechnology in terms of colloidal synthesis, general understanding of nanocrystal growth mechanisms, and their impact in a wide range of applications. They exhibit strong visible colors due to localized surface plasmon resonance (LSPR) that depends on their size, shape, composition, and the surrounding dielectric environment. Under resonant excitation, the LSPR of plasmonic NPs leads to a strong field enhancement near their surfaces and thus enhances various light-matter interactions. These unique optical properties of plasmonic NPs have been used to design chemical and biological sensors. Over the last few decades, colloidal plasmonic NPs have been greatly exploited in sensing applications through LSPR shifts (colorimetry), surface-enhanced Raman scattering, surface-enhanced fluorescence, and chiroptical activity. Although colloidal plasmonic NPs have emerged at the forefront of nanobiosensors, there are still several important challenges to be addressed for the realization of plasmonic NP-based sensor kits for routine use in daily life. In this comprehensive review, researchers of different disciplines (colloidal and analytical chemistry, biology, physics, and medicine) have joined together to summarize the past, present, and future of plasmonic NP-based sensors in terms of different sensing platforms, understanding of the sensing mechanisms, different chemical and biological analytes, and the expected future technologies. This review is expected to guide the researchers currently working in this field and inspire future generations of scientists to join this compelling research field and its branches.

Multifunctional gold nanoparticles for cancer theranostics

The diagnosis and treatment of cancer can often be challenging requiring more attractive options. Some types of cancers are more aggressive than others and symptoms for many cancers are subtle, especially in the early stages. Nanotechnology provides high sensitivity, specificity and multimodal capability for cancer detection, treatment and monitoring. In particular, metal nanoparticles (NPs) such as gold nanoparticles (AuNPs) are attractive nanosystems for researchers interested in bioimaging and therapy. The size, shape and surface of AuNPs can be modified for improving targeting and accumulation in cancer cells, for example through introduction of ligands and surface charge. The interactions of AuNPs with electromagnetic radiation (e.g., visible-near-infrared, X-rays) can be used for photothermal therapy and radiation therapy, through heat generated from light absorption and emission of Auger electrons, respectively. The subsequent expansion and high X-ray attenuation from AuNPs can be used for enhancing contrast for tumor detection (e.g., using photoacoustic, computed tomography imaging). Multi-functionality can be further extended through covalent/non-covalent functionalization, for loading additional imaging/therapeutic molecules for combination therapy and multimodal imaging. In order to cover the important aspects for designing and using AuNPs for cancer theranostics, this review focuses on the synthesis, functionalization and characterization methods that are important for AuNPs, and presents their unique properties and different applications in cancer theranostics.

Chaperone/Polymer Complexation of Protein-Based Fluorescent Nanoclusters against Silica Encapsulation-Induced Physicochemical Stresses

Silica encapsulation under ambient conditions is commonly used to shield protein-based nanosystems from chemical stress. However, encapsulation-induced photo- and structural instabilities at elevated temperatures have been overlooked. Using bovine serum albumin-capped fluorescent gold nanoclusters (BSA-AuNCs) as a model, we demonstrated that chaperone/polymer layer-by-layer complexation can stabilize the template to resist encapsulation-induced fragmentation/reorganization and emission increases at 37 °C or higher temperatures. We first wrapped BSA-AuNCs with α-crystallin chaperones (α-Crys) to gain the highest thermal stability at a 1:50 molar ratio and then enfolded BSA-AuNC/α-Crys with thermoresponsive poly-N-isopropylacrylamide (PNIPAM) at 60 °C to shield silica interaction and increase the chaperone-client protein accessibility. The resulting BSA-AuNC/α-Crys/PNIPAM (BαP) was encapsulated by a sol-gel process to yield BαP-Si (∼80 ± 4.5 nm), which exhibited excellent structural integrity and photostability against chemical and thermal stresses. Moreover, targeted BαP-Si demonstrated prolonged fluorescence stability for cancer cell imaging. This template stabilization strategy for silica encapsulation is biocompatible and applicable to other protein-based nanosystems.

Comparison of Survivin Determination by Surface-Enhanced Fluorescence and Raman Spectroscopy on Nanostructured Silver Substrates

Survivin belongs to a family of proteins that promote cellular proliferation and inhibit cellular apoptosis. Its overexpression in various cancer types has led to its recognition as an important marker for cancer diagnosis and treatment. In this work, we compare two approaches for the immunochemical detection of survivin through surface-enhanced fluorescence or Raman spectroscopy using surfaces with nanowires decorated with silver nanoparticles in the form of dendrites or aggregates as immunoassays substrates. In both substrates, a two-step non-competitive immunoassay was developed using a pair of specific monoclonal antibodies, one for detection and the other for capture. The detection antibody was biotinylated and combined with streptavidin labeled with rhodamine for the detection of surface-enhanced fluorescence, while, for the detection via Raman spectroscopy, streptavidin labeled with peroxidase was used and the signal was obtained after the application of 3,3',5,5'-tetramethylbenzidine (TMB) precipitating substrate. It was found that the substrate with the silver dendrites provided higher fluorescence signal intensity compared to the substrate with the silver aggregates, while the opposite was observed for the Raman signal. Thus, the best substrate was used for each detection method. A detection limit of 12.5 pg/mL was achieved with both detection approaches along with a linear dynamic range up to 500 pg/mL, enabling survivin determination in human serum samples from both healthy and ovarian cancer patients for cancer diagnosis and monitoring purposes.

Plasmonic Cross-Reactive Sensing Noses and Tongues

The advancements in the capabilities of artificial sensory technologies, such as electronic/optical noses and tongues, have significantly enhanced their ability to identify complex mixtures of analytes. These improvements are rooted in the evolving manufacturing processes of cross-reactive sensor arrays (CRSAs) and the development of innovative computational methods. The potential applications in early diagnosis, food quality control, environmental monitoring, and more, position CRSAs as an exciting area of research for scientists from diverse backgrounds. Among these, plasmonic CRSAs are particularly noteworthy because they offer enhanced capabilities for remote, fast, and even real-time monitoring, in addition to better portability of instrumentation. Specifically, the synergy between the localized surface plasmon resonance (LSPR) of metallic nanoparticles (NPs) and CRSAs introduces advanced techniques such as LSPR, metal-enhanced fluorescence (MEF), surface-enhanced infrared absorption (SEIRA), surface-enhanced Raman scattering (SERS), and surface-enhanced resonance Raman scattering (SERRS) spectroscopies. This review delves into the importance and versatility of optical-CRSAs, especially those based on plasmonic materials, discussing recent applications and potential new research directions.

Simultaneous ionic cobalt sensing and toxic Congo red dye removal: a circular economic approach involving silver-enhanced fluorescence

A highly fluorescent quinone-capped silver hydrosol (AgOSA) was obtained using salicylaldehyde and an ionic silver solution. Such metal-enhanced fluorescence was efficiently quenched with Congo red dye (CR), producing CRAgOSA, due to the strong silver-sulfur interaction, replacing the capping of quinone (oxidized salicylaldehyde). The introduction of cobalt ions restored the fluorescence by engaging CR (CoCRAgOSA). Cobalt-induced fluorescence enhancement was 8.3 times higher than that of AgOSA due to the freeing of CR and the release of self-quenching of excess quinone molecules in CoCRAgOSA. The mammoth and selective fluorescence enhancement with ionic cobalt assisted in designing a turn-on ionic cobalt sensor with a limit of detection (LOD) of 9.4 × 10-11 M and a linear detection range (5 × 10-5 to 10-9 M). Moreover, toxic CR dye was eliminated by quinone-capped silver nanoparticles and Co2+ due to chemisorption. Not only the fluorimetric sensing of ionic cobalt but also the colorimetric sensing of Hg2+ was designed due to the simultaneous aggregation of AgNPs and complexation with CR induced by Hg2+ (LOD 1.36 × 10-5 M and linear detection range from 1.00 × 10-4 to 5 × 10-7 M). We applied our sensing method to estimate ionic cobalt and mercury in natural samples. The experiment was a unique case of circular economy, where a toxic dye was used for making a nanosensor.

A "Talking" between Gold Nanoparticle and a Luminescent Iridium(III) Complex: A Study of the Effect Due to the Interaction between Plasmon Resonance and a Fluorophore

This paper explores a novel synthesis and characterization of silica-coated gold nanorods (AuNRs) embedding a highly emissive cyclometalated iridium(III) complex, denoted as Ir1. We investigate the optical properties and the interplay between the metal compound and gold plasmon, observing how the emission of Ir1 incorporated into the nanoparticles shows two emission bands, one in the blue and the other in the green-orange range of the visible spectrum. To obtain a clearer picture of what we were observing, we synthesized analogous nanosystems, from which it was possible to highlight the effect of different features. Based on what we observed, we proposed that the fraction of the iridium(III) complex in direct contact with the surface of the gold nanoparticle undergoes a "demixing" of the excited state, which, for cyclometalated iridium complexes, is generally considered a mixed LC+MLCT state. This preliminary study sheds light on the complexity of the "talking" between a fluorophore and a plasmonic system, highlighting the importance of considering the emitter typology when modeling such systems.

Silver Nanoparticles Improve Fluorophore Photostability: Application to a Hypericin Study

Protection against the negative effects of solar radiation involves using cosmetics with a UV filter, but visible radiation can also have negative effects. We use dietary supplements and take medications; unfortunately, many of them contain substances that degrade under the influence of visible light, which transform into chemical compounds harmful to health. Manufacturers often include information on the prohibition of exposure to sunlight on the packaging, but consumers often do not read the product leaflet. The solution to this problem may be the addition of silver particles to preparations. In the presented article, we proposed the use of silver nanoparticles to reduce the photobleaching and photoreaction of fluorophore, while increasing the fluorescence intensity. For our research, we used a compound that is particularly sensitive to radiation: hypericin.

Tailor-Made Gold Nanomaterials for Applications in Soft Bioelectronics and Optoelectronics

In modern nanoscience and nanotechnology, gold nanomaterials are indispensable building blocks that have demonstrated a plethora of applications in catalysis, biology, bioelectronics, and optoelectronics. Gold nanomaterials possess many appealing material properties, such as facile control over their size/shape and surface functionality, intrinsic chemical inertness yet with high biocompatibility, adjustable localized surface plasmon resonances, tunable conductivity, wide electrochemical window, etc. Such material attributes have been recently utilized for designing and fabricating soft bioelectronics and optoelectronics. This motivates to give a comprehensive overview of this burgeoning field. The discussion of representative tailor-made gold nanomaterials, including gold nanocrystals, ultrathin gold nanowires, vertically aligned gold nanowires, hard template-assisted gold nanowires/gold nanotubes, bimetallic/trimetallic gold nanowires, gold nanomeshes, and gold nanosheets, is begun. This is followed by the description of various fabrication methodologies for state-of-the-art applications such as strain sensors, pressure sensors, electrochemical sensors, electrophysiological devices, energy-storage devices, energy-harvesting devices, optoelectronics, and others. Finally, the remaining challenges and opportunities are discussed.

A naphthalimide-based portable fluorescent sensor integrated with a photoelectric converter for rapid and on-site detection of type II pyrethroids in celery

The on-site detection of pyrethroids, particularly type II pyrethroids, remains a challenging task in complex vegetable samples. Herein, a novel method based on naphthalimide was developed to realize the specific detection of type II pyrethroids by hydrolyzing and utilizing the compound m-phenoxybenzaldehyde (3-PBD). Hydrazine group, used as the appropriate moiety, was introduced into the fluorescent dye 1,8-naphthalimide to construct the fluoroprobe NAP. In the presence of 3-PBD, NAP displayed the prominently enhanced fluorescence and also exhibited high selectivity. This proposed method exhibited high anti-inference effects in complex media, realizing sensitive detection of 3-PBD with linear range of 2.15-800 μM and a low detection limit (LOD) of 0.64 μM. The underlying fluorescence-responsive mechanisms were in-depth elucidated by combining spectral analyses with TD-DFT theoretical calculations. Additionally, a direct and rapid hydrolysis method for deltamethrin in celery was established, achieving a high hydrolysis efficiency of >90% within 15 min. Furthermore, a portable fluorescence sensor (PFS) was developed based on high-power LEDs and photodetectors. PFS supplied a LOD of 2.23 μM for 3-PBD and exhibited comparable stability by a fluorescence spectrometer when detecting celery hydrolysate. Moreover, external power source is not required for PFS operations, thereby enabling rapid and on-site detection by transmitting data to a smartphone via bluetooth. These findings extend the academic knowledge in the field of specific pyrethroids detection and contribute to the development of on-site methods for pesticide residual analyses in food matrices.

Plasmonic nanoparticle-assisted single-molecule dynamic binding for protein kinase activity digital counting

In this work, we design a new approach for digital counting-based protein kinase activity assay by using plasmonic nanoparticle-assisted single-molecule dynamic binding. This method breaks the concentration-dependent limitation in single-molecule detection and displays good selectivity and sensitivity with a detection limit as low as 0.0001 U mL-1 for the protein kinase A (PKA) assay, which would find broad applications in drug screening and medical investigations.

Applications of Metals and Metal Compounds in Improving the Sensitivity of Microfluidic Biosensors - A Review

The enhancement of detection sensitivity in microfluidic sensors has been a continuously explored field. Initially, many strategies for sensitivity improvement involved introducing enzyme cascade reactions, but enzyme-based reactions posed challenges in terms of cost, stability, and storage. Therefore, there is an urgent need to explore enzyme-free cascade amplification methods, which are crucial for expanding the application range and improving detection stability. Metal or metal compound nanomaterials have gained great attention in the exploitation of microfluidic sensors due to their ease of preparation, storage, and lower cost. The unique physical properties of metallic nanomaterials, including surface plasmon resonance, surface-enhanced Raman scattering, metal-enhanced fluorescence, and surface-enhanced infrared absorption, contribute significantly to enhancing detection capabilities. The metal-based catalytic nanomaterials, exemplified by Fe3O4 nanoparticles and metal-organic frameworks, are considered viable alternatives to biological enzymes due to their excellent performance. Herein, we provide a detailed overview of the applications of metals and metal compounds in improving the sensitivity of microfluidic biosensors. This review not only highlights the current developments but also critically analyzes the challenges encountered in this field. Furthermore, it outlines potential directions for future research, contributing to the ongoing development of microfluidic biosensors with improved detection sensitivity.

Recent applications of coinage metal nanoparticles passivated with salicylaldehyde and salicylaldehyde-based Schiff bases

Salicylaldehyde (SD) and its derivatives are effective precursors for generating coinage metal (gold, silver, and copper) nanoparticles (NPs). These NPs have a variety of potential environmental applications, such as in water purification and sensing, and those arising from their antibacterial activity. The use of SD and its derivatives for synthesizing coinage NPs is attractive due to several factors. First, SD is a relatively inexpensive and readily available starting material. Second, the synthetic procedures are typically simple and can be carried out under mild conditions. Finally, the resulting NPs can be tailored to have specific properties, such as size, shape, and surface functionality, by varying the reaction conditions. In an alkaline solution, the phenolate form of SD was converted to its quinone form, while ionic coinage metal salts were converted to zero-valent nanoparticles. The capping in situ produced quinone of coinage metal nanoparticles generated metal-enhanced fluorescence under suitable experimental conditions. The formation of iminic bonds during the formation of Schiff bases altered the properties (especially metal-enhanced fluorescence) and applications.

Magnesium Nanoparticles for Surface-Enhanced Raman Scattering and Plasmon-Driven Catalysis

Nanostructures of some metals can sustain localized surface plasmon resonances, collective oscillations of free electrons excited by incident light. This effect results in wavelength-dependent absorption and scattering, enhancement of the incident electric field at the metal surface, and generation of hot carriers as a decay product. The enhanced electric field can be utilized to amplify the spectroscopic signal in surface-enhanced Raman scattering (SERS), while hot carriers can be exploited for catalytic applications. In recent years, cheaper and more earth abundant alternatives to traditional plasmonic Au and Ag have gained growing attention. Here, we demonstrate the ability of plasmonic Mg nanoparticles to enhance Raman scattering and drive chemical transformations upon laser irradiation. The plasmonic properties of Mg nanoparticles are characterized at the bulk and single particle level by optical spectroscopy and scanning transmission electron microscopy coupled with electron energy-loss spectroscopy and supported by numerical simulations. SERS enhancement factors of ∼102 at 532 and 633 nm are obtained using 4-mercaptobenzoic acid and 4-nitrobenzenethiol. Furthermore, the reductive coupling of 4-nitrobenzenethiol to 4,4'-dimercaptoazobenzene is observed on the surface of Mg nanoparticles under 532 nm excitation in the absence of reducing agents, indicating a plasmon-driven catalytic process. Once decorated with Pd, Mg nanostructures display an enhancement factor of 103 along with an increase in the rate of catalytic coupling. The results of this study demonstrate the successful application of plasmonic Mg nanoparticles in sensing and plasmon-enhanced catalysis.

Recent advances towards point-of-care devices for fungal detection: Emphasizing the role of plasmonic nanomaterials in current and future technologies

Fungal infections are a significant global health problem, particularly affecting individuals with weakened immune systems. Moreover, as uncontrolled antibiotic and immunosuppressant use increases continuously, fungal infections have seen a dramatic increase, with some strains developing antibiotic resistance. Traditional approaches to identifying fungal strains often rely on morphological characteristics, thus owning limitations, such as struggles in identifying several strains or distinguishing between fungal strains with similar morphologies. This review explores the multifaceted impact of fungi infections on individuals, healthcare providers, and society, highlighting the often-underestimated economic burden and healthcare implications of these infections. In light of the serious constraints of traditional fungal identification methods, this review discusses the potential of plasmonic nanoparticle-based biosensors for fungal infection identification. These biosensors can enable rapid and precise fungal pathogen detection by exploiting several readout approaches, including various spectroscopic techniques, colorimetric and electrochemical assays, as well as lateral-flow immunoassay methods. Moreover, we report the remarkable impact of plasmonic Lab on a Chip technology and microfluidic devices, as they recently emerged as a class of advanced biosensors. Finally, we provide an overview of smartphone-based Point-of-Care devices and the associated technologies developed for detecting and identifying fungal pathogens.

On-chip bioluminescence biosensor for the detection of microbial surface contamination

Detection of microbial pathogens is important for food safety reasons, and for monitoring sanitation in laboratory environments and health care settings. Traditional detection methods such as culture-based and nucleic acid-based methods are time-consuming, laborious, and require expensive laboratory equipment. Recently, ATP-based bioluminescence methods were developed to assess surface contamination, with commercial products available. In this study, we introduce a biosensor based on a CMOS image sensor for ATP-mediated chemiluminescence detection. The original lens and IR filter were removed from the CMOS sensor revealing a 12 MP periodic microlens/pixel array on an area of 6.5 mm × 3.6 mm. UltraSnap swabs are used to collect samples from solid surfaces including personal electronic devices, and office and laboratory equipment. Samples mixed with chemiluminescence reagents were placed directly on the surface of the image sensor. Close proximity of the sample to the photodiode array leads to high photon collection efficiency. The population of microorganisms can be assessed and quantified by analyzing the intensity of measured chemiluminescence. We report a linear range and limit of detection for measuring ATP in UltraSnap buffer of 10-1000 nM and 225 fmol, respectively. The performance of the CMOS-based device was compared to a commercial luminometer, and a high correlation with a Pearson's correlation coefficient of 0.98589 was obtained. The Bland-Altman plot showed no significant bias between the results of the two methods. Finally, microbial contamination of different surfaces was analyzed with both methods, and the CMOS biosensor exhibited the same trend as the commercial luminometer.

"Turn-on" and pinhole-free ultrathin core-shell Au@SiO2 nanoparticle-based metal-enhanced fluorescent (MEF) chemodosimeter for Hg2

This study reports a metal-enhanced fluorescence chemodosimeter for highly sensitive detection of Hg2+ ions. Silica-coated Au nanoparticles (Au@SiO2 NPs) with a pinhole-free 4-5 nm shell were synthesized and functionalized with a monolayer of turn-on fluorescent probes. Compared to other organic fluorescent probes suffering from poor biocompatibility and detection limits, this design of a monolayer of turn-on fluorescent probes immobilized on the Au@SiO2 NPs with a pinhole-free 4-5 nm shell avoids fluorescence quenching and allows the fluorescent probe within the field of the inner Au NPs to experience metal-enhanced fluorescence. With this design, the chemodosimeter permits fluorescence emission in the presence of Hg2+ ions, because they trigger the ring-opening reaction of the fluorescent probe immobilized on the Au@SiO2 NPs. Additionally, the fluorescent probe is distanced by the thin SiO2 shell from directly attaching to the metallic Au NPs, which not only avoids fluorescence quenching but allows the fluorescent probe within the long-ranged field of the inner Au NPs to experience metal-enhanced fluorescence. As a result, the detection limit for the chemodosimeter can reach up to 5.0 × 10-11 M, nearly two orders of magnitude higher than that achieved for the free fluorescent probe. We also demonstrate the acquisition of images of Hg2+ in HTC116 cells and zebrafish using a simple fluorescence confocal imaging technique. The fluorescence response results for HTC116 cells and zebrafish show that the probes can permeate into cells and organisms. Considering the availability of the many organic fluorescent probes that have been designed, the current designed metal-enhanced fluorescence chemodosimeter holds great potential for fluorescence detection of diverse species and fluorescence imaging.

Electropolymerization of Preferred-Oriented Conjugated Microporous Polymer Films for Enhanced Fluorescent Sensing

High-quality conjugated microporous polymer (CMP) films with orientation and controlled structure are extremely desired for applications. Here, we report the effective construction of CMP 3D composite films (pZn/PTPCz) with a controlled porosity structure and preferred orientation using the template-assisted electropolymerization (EP) approach for the first time. The structure of pZn/PTPCz composite thin films and nitrophenol sensing performance were thoroughly studied. When compared to the control CMP film made on flat indium tin oxide (ITO) substrates, the as-prepared pZn/PTPCz composite films showed significantly enhanced fluorescent intensity and much better sensing performance for the model explosive. This was attributed to the metal-enhanced fluorescence (MEF) of porous nanostructured zinc (pZn) and the additional macroporosity of the pZn/PTPCz composite films. This work provides a feasible approach for creating oriented 3D CMP-based thin films for advanced applications.

Plasmonic Fluorescence Sensors in Diagnosis of Infectious Diseases

The increasing demand for rapid, cost-effective, and reliable diagnostic tools in personalized and point-of-care medicine is driving scientists to enhance existing technology platforms and develop new methods for detecting and measuring clinically significant biomarkers. Humanity is confronted with growing risks from emerging and recurring infectious diseases, including the influenza virus, dengue virus (DENV), human immunodeficiency virus (HIV), Ebola virus, tuberculosis, cholera, and, most notably, SARS coronavirus-2 (SARS-CoV-2; COVID-19), among others. Timely diagnosis of infections and effective disease control have always been of paramount importance. Plasmonic-based biosensing holds the potential to address the threat posed by infectious diseases by enabling prompt disease monitoring. In recent years, numerous plasmonic platforms have risen to the challenge of offering on-site strategies to complement traditional diagnostic methods like polymerase chain reaction (PCR) and enzyme-linked immunosorbent assays (ELISA). Disease detection can be accomplished through the utilization of diverse plasmonic phenomena, such as propagating surface plasmon resonance (SPR), localized SPR (LSPR), surface-enhanced Raman scattering (SERS), surface-enhanced fluorescence (SEF), surface-enhanced infrared absorption spectroscopy, and plasmonic fluorescence sensors. This review focuses on diagnostic methods employing plasmonic fluorescence sensors, highlighting their pivotal role in swift disease detection with remarkable sensitivity. It underscores the necessity for continued research to expand the scope and capabilities of plasmonic fluorescence sensors in the field of diagnostics.

Photoluminescence from FRET pairs coupled with Mie-resonant silicon nanospheres

Optically resonant nanoparticles decorated with donor-acceptor molecular pairs have been attracting attention for applications as nanoprobes in bioimaging and biosensing. We produced composite nanoparticles composed of donor-acceptor molecular pairs and silicon nanospheres (Si NSs) with diameters of 100-200 nm exhibiting Mie resonances in the visible range and studied the effect of Mie resonances on their photoluminescence properties. We showed that the photoluminescence spectra are strongly modified by Mie resonances and the spectral shape is controlled in a wide range by the Si NS size; by controlling the size, we can achieve the photoluminescence maximum from that of a donor molecule to that of an acceptor molecule almost continuously. From the photoluminescence decay properties in combination with theoretical calculations, we showed that the observed strong modification of the spectral shape is mainly due to the Purcell effect on donor and acceptor molecules, and the effect of Mie resonances on the Förster resonance energy transfer (FRET) rate is relatively small. We also showed that because of the large Purcell effect and the small FRET rate enhancement, Mie resonances decrease the FRET efficiency.

Construction of nanoparticle-on-mirror nanocavities and their applications in plasmon-enhanced spectroscopy

Plasmonic nanocavities exhibit exceptional capabilities in visualizing the internal structure of a single molecule at sub-nanometer resolution. Among these, an easily manufacturable nanoparticle-on-mirror (NPoM) nanocavity is a successful and powerful platform for demonstrating various optical phenomena. Exciting advances in surface-enhanced spectroscopy using NPoM nanocavities have been developed and explored, including enhanced Raman, fluorescence, phosphorescence, upconversion, etc. This perspective emphasizes the construction of NPoM nanocavities and their applications in achieving higher enhancement capabilities or spatial resolution in dark-field scattering spectroscopy and plasmon-enhanced spectroscopy. We describe a systematic framework that elucidates how to meet the requirements for studying light-matter interactions through the creation of well-designed NPoM nanocavities. Additionally, it provides an outlook on the challenges, future development directions, and practical applications in the field of plasmon-enhanced spectroscopy.

DNA and Nanomaterials: A Functional Combination for DNA Sensing

Recent decades have experienced tough situations due to the lack of reliable diagnostic facilities. The most recent cases occurred during the pandemic, where researchers observed the lack of diagnostic facilities with precision. Microorganisms and viral disease's ability to escape diagnosis has been a global challenge. DNA always has been a unique moiety with a strong and precise base-paired structure. DNA in human and foreign particles makes identification possible through base pairing. Since then, researchers have focused heavily on designing diagnostic assays targeting DNA in particular. Moreover, DNA nanotechnology has contributed vastly to designing composite nanomaterials by combining DNA/nucleic acids with functional nanomaterials and inorganic nanoparticles exploiting their physicochemical properties. These nanomaterials often exhibit unique or enhanced properties due to the synergistic activity of the many components. The capabilities of DNA and additional nanomaterials have shown the combination of robust and advanced tailoring of biosensors. Preceding findings state that the conventional strategies have exhibited certain limitations such as a low range of target detection, less biodegradability, subordinate half-life, and high susceptibility to microenvironments; however, a DNA-nanomaterial-based biosensor has overcome these limitations meaningfully. Additionally, the unique properties of nucleic acids have been studied extensively due to their high signal conduction abilities. Here, we review recent studies on DNA-nanomaterial-based biosensors, their mechanism of action, and improved/updated strategies in vivo and in situ. Furthermore, this review highlights the recent methodologies on DNA utilization to exploit the interfacial properties of nanomaterials in DNA sensing. Lastly, the review concludes with the limitations/challenges and future directions.

17β-estradiol biosensors based on different bioreceptors and their applications

17β-Estradiol (E2) is a critical sex steroid hormone, which has significant effects on the endocrine systems of both humans and animals. E2 is also believed to play neurotrophic and neuroprotective roles in the brain. Biosensors present a powerful tool to detect E2 because of their small, efficient, and flexible design. Furthermore, Biosensors can quickly and accurately obtain detection results with only a small sampling amount, which greatly meets the detection of the environment, food safety, medicine safety, and human body. This review focuses on previous studies of biosensors for detecting E2 and divides them into non-biometric sensors, enzyme biosensors, antibody biosensors, and aptamer biosensors according to different bioreceptors. The advantages, disadvantages, and design points of various bioreceptors for E2 detection are analyzed and summarized. Additionally, applications of different bioreceptors of E2 detection are presented and highlight the field of environmental monitoring, food and medicine safety, and disease detection in recent years. Finally, the development of E2 detection by biosensor is prospected.

Fluorogen-Functionalized Mesoporous Silica Hybrid Sensing Materials: Applications in Cu2+ Detection

Since Cu2+ ions play a pivotal role in both ecosystems and human health, the development of a rapid and sensitive method for Cu2+ detection holds significant importance. Fluorescent mesoporous silica materials (FMSMs) have garnered considerable attention in the realm of chemical sensing, biosensing, and bioimaging due to their distinctive structure and easily functionalized surfaces. As a result, numerous Cu2+ sensors based on FMSMs have been devised and extensively applied in environmental and biological Cu2+ detection over the past few decades. This review centers on the recent advancements in the methodologies for preparing FMSMs, the mechanisms underlying sensing, and the applications of FMSMs-based sensors for Cu2+ detection. Lastly, we present and elucidate pertinent perspectives concerning FMSMs-based Cu2+ sensors.

High index dielectric films on metals: An island of emission

Fluorescent emitters are quenched near the surfaces of metals via rapid energy transfer to the metal, via surface plasmons, waveguide modes, and absorption. Commonly, this quenching is reduced by introducing a polymeric or dielectric spacer but requires large distances, at least a fraction of the wavelength, between the metal and chromophore. Using the classical theory for a dipole above a metal/dielectric substrate, we investigate the fluorescent yield for emitters above a wide range of metals and spacers. For metals with low loss and low plasma frequencies, a high index spacer is shown to be advantageous for obtaining higher fluorescent yield in an "island of emission" at finely tuned spacer thickness just 20-30 nm from the metal surface. For such metal-dielectric combinations, fluorophores can be placed surprisingly close to the metal surface while remaining significantly emissive.

Fluorophore Interactions with the Surface Modes and Internal Modes of a Photonic Crystal

The metal-ligand complex tris(2,2'-bipyridine)ruthenium(II) chloride (Ru probe) displays a broad emission spectrum ranging from 540 to 730 nm. The emission spectra of Ru probe were measured when placed on top of a one-dimensional photonic crystal (1DPC), which supports both Bloch surface wave (BSW) and internal modes for wavelengths below 640 nm and only internal modes above 640 nm. The S-polarized emission spectra, with the electric vector parallel to the 1DPC surface, were found to be strongly dependent on the observation angle through the coupling prism. Also, the usual single broad-emission spectrum of Ru probe on glass was converted into two or more narrow-band-spectrum on the 1DPC, with emission band maxima dependent on the observation angle. The two S-polarized emission band peaks for Ru probe were found to be consistent with coupling to the BSW and first internal mode (IM1) of the 1DPC. The same spectral shifts and changes in emission maxima were observed by using Kretschmann and reverse Kretschmann illuminations. As the coupling requires the emitter to be in proximity with the photonic structure, we calculated near- and far-field distributions of a dipole directly located on the 1DPC surface. Finite-Difference Time-Domain (FDTD) simulations were performed to confirm fluorophore coupling to the BSW and internal modes (IMs). Both the measured and simulated results showed that IM coupled emission is significant. Coupling to the IM mode occurred at longer wavelengths where the 1DPC did not support a BSW. These results demonstrate that a simple Bragg grating, without a BSW mode, can be used for detection of surface-bound fluorophores.

Advances in three dimensional metal enhanced fluorescence based biosensors using metal nanomaterial and nano-patterned surfaces

Metal enhanced fluorescence (MEF) is a phenomenon that increases fluorescence signal through placement of metal near a fluorophore. For biosensing applications, MEF-based biosensors are becoming increasingly popular as it enables highly sensitive detection of molecules, important for early diagnosis. The structure and size of the metal influence the optical properties through enhancing the fluorophore photostability and light absorption and emission. In recent years, many metal nanostructures have been fabricated and examined for their effectiveness in developing MEF-based biosensors. This review focuses on the latest applications of three-dimensional nanostructures and nano-patterned surfaces used to develop and improve fluorescence sensing via MEF. Current reviews mostly discussed the applications of two dimensional MEF and metal-nanoparticles-based MEF with a focus on fabrication of nanoparticles and metal substrates. In this article, we focused more on the effect of the metal nanostructure and size on MEF and then provided an in-depth summary of the performance of the state-of-the-art three dimensional MEF-based biosensors. While more work is needed to demonstrate applicability for complex samples, it is evident that with the use of metal nanoparticles and three dimensional nano-patterns, the assay sensitivity of fluorescence-based detection can be greatly improved, making it suitable for use in early disease diagnostics.

Microfluidic devices integrated with plasmonic nanostructures for sensitive fluorescent immunoassays

The robust identification and quantification of various biomarkers is of utmost significance in clinical diagnostics and precision medicine. Fluorescent immunoassays are widely used and considered as a gold standard for biomarker detection due to their high specificity and accuracy. However, current commercial immunoassay tests suffer from limited detection sensitivity and complicated, labor-intensive operation procedures, making them impractical for point-of-care diagnosis, particularly in resource-limited regions. Recently, microfluidic immunoassay devices integrated with plasmonic nanostructures have emerged as a powerful tool for sensitive detection of biomarkers, addressing specific issues, such as integration schemes, easy operation, multiplexed detection, and sensitivity enhancement. In this paper, we provide a discussion on the recent advances in the plasmonic nanostructures integrated with microfluidic devices for fluorescent immunoassays. We shed light on the nanofabrication strategies and various fluidic designs for rapid, sensitive, and highly efficient sensing of antigens. Finally, we share our perspectives on the potential directions of these integrated devices for practical applications.

Chiral Plasmonic Hybrid Nanostructures: A Gateway to Advanced Chiroptical Materials

Chirality introduces a new dimension of functionality to materials, unlocking new possibilities across various fields. When integrated with plasmonic hybrid nanostructures, this attribute synergizes with plasmonic and other functionalities, resulting in unprecedented chiroptical materials that push the boundaries of the system's capabilities. Recent advancements have illuminated the remarkable chiral light-matter interactions within chiral plasmonic hybrid nanomaterials, allowing for the harnessing of their tunable optical activity and hybrid components. These advancements have led to applications in areas such as chiral sensing, catalysis, and spin optics. Despite these promising developments, there remains a need for a comprehensive synthesis of the current state-of-the-art knowledge, as well as a thorough understanding of the construction techniques and practical applications in this field. This review begins with an exploration of the origins of plasmonic chirality and an overview of the latest advancements in the synthesis of chiral plasmonic hybrid nanostructures. Furthermore, representative emerging categories of hybrid nanomaterials are classified and summarized, elucidating their versatile applications. Finally, the review engages with the fundamental challenges associated with chiral plasmonic hybrid nanostructures and offer insights into the future prospects of this advanced field.

Innovative, Flexible, and Miniaturized Microfluidic Paper-Based Plasmonic Chip for Efficient Near-Infrared Metal Enhanced Fluorescence Biosensing and Imaging

The implementation of metal enhanced fluorescence (MEF) as an efficient detection tool, especially in the near-infrared region of the electromagnetic spectrum, is a rather new direction for diagnostic analytical technologies. In this context, we propose a novel microfluidic plasmonic design based on paper for efficient MEF detection of the "proof-of-concept" biotin-streptavidin recognition interaction. Our design made use of the benefits of gold nanobipyramids (AuBPs), considering the strong enhanced electromagnetic field present at their sharp tips, and filter paper to operate as a natural microfluidic channel due to excellent wicking abilities. The calligraphed plasmonic paper, obtained using a commercial pen filled with AuBPs, was integrated in a robust sandwich optically transparent polydimethylsiloxane chip, exhibiting portability and flexibility while preserving the chip's properties. To place the Alexa 680 fluorophore at an optimal distance from the nanobipyramid substrate, the human IgG-anti-IgG-conjugated biotin sandwich reaction was employed. Thus, upon the capture of Alexa 680-conjugated streptavidin by the biotinylated system, a 1.3-fold average enhancement of the fluorophore's emission was determined by bulk fluorescence measurements. However, the local enhancement factor was considerably higher with values spanning from 5 to 6.3, as proven by mapping the fluorescence emission under both re-scan microscopy and fluorescence lifetime imaging, endorsing the proposed chip's feasibility for bulk MEF biosensing as well as high-resolution MEF bioimaging. Finally, the versatility of our chip was demonstrated by adapting the biosensing protocol for cardiac troponin I biomarker detection, validated using 10 plasma samples collected from pediatric patients and corroborated with a conventional ELISA assay.

Sensitive and Portable Signal Readout Strategies Boost Point-of-Care CRISPR/Cas12a Biosensors

Point-of-care (POC) detection is getting more and more attention in many fields due to its accuracy and on-site test property. The CRISPR/Cas12a system is endowed with excellent sensitivity, target identification specificity, and signal amplification ability in biosensing because of its unique trans-cleavage ability. As a result, a lot of research has been made to develop CRISPR/Cas12a-based biosensors. In this review, we focused on signal readout strategies and summarized recent sensitivity-improving strategies in fluorescence, colorimetric, and electrochemical signaling. Then we introduced novel portability-improving strategies based on lateral flow assays (LFAs), microfluidic chips, simplified instruments, and one-pot design. In the end, we also provide our outlook for the future development of CRISPR/Cas12a biosensors.

Bimetallic copper palladium nanorods: plasmonic properties and palladium content effects

Cu is an inexpensive alternative plasmonic metal with optical behaviour comparable to Au but with much poorer environmental stability. Alloying with a more stable metal can improve stability and add functionality, with potential effects on the plasmonic properties. Here we investigate the plasmonic behaviour of Cu nanorods and Cu-CuPd nanorods containing up to 46 mass percent Pd. Monochromated scanning transmission electron microscopy electron energy-loss spectroscopy first reveals the strong length dependence of multiple plasmonic modes in Cu nanorods, where the plasmon peaks redshift and narrow with increasing length. Next, we observe an increased damping (and increased linewidth) with increasing Pd content, accompanied by minimal frequency shift. These results are corroborated by and expanded upon with numerical simulations using the electron-driven discrete dipole approximation. This study indicates that adding Pd to nanostructures of Cu is a promising method to expand the scope of their plasmonic applications.

A near-infrared fluorescence-enhancing plasmonic biosensing microarray identifies soluble PD-L1 and ICAM-1 as predictive checkpoint biomarkers for cancer immunotherapy

Sensitive and accurate biomarker-driven assay guidance has been widely adopted to identify responsive patients for immune checkpoint blockade (ICB) therapy to impede disease progression and extend survival. However, most current assays are invasive, requiring surgical pathology specimens and only informing monochronic information. Here, we report a multiplexed enhanced fluorescence microarray immunoassay (eFMIA) based on a nanostructured gold nanoisland substrate (AuNIS), which macroscopically amplifies near-infrared fluorescence (NIRF) of a structurally symmetric IRDye78 fluorophore by over two orders of magnitude of 202.6-fold. Aided by non-contact piezo-driven micro-dispensing (PDMD), eFMIA simultaneously and semi-quantitatively detected intracellular and secreted programmed death-ligand 1 (PD-L1) and intercellular adhesion molecule-1 (ICAM-1) in human nasopharyngeal carcinoma (NPC) cells. The assay performance was superior to fluorescence immunoassays (FIA) and enzyme-linked immunosorbent assays (ELISA), with lower detection limits. Using eFMIA, we found significantly differential levels of soluble PD-L1 (sPD-L1) and sICAM-1 in the sera of 28 cancer patients, with different clinical outcomes following anti-PD-1 ICB therapy. With a well-characterized mechanism, the high-performance plasmonic multiplexed assay with the composite biomarkers may be a valuable tool to assist clinicians with decision-making and patient stratification to afford predictive ICB therapy responses.

Luminescent Probe Based on Terbium-Carbon Quantum Dots for the Quantification of Imidacloprid in Caneberries

We propose a modification of terbium-sensitized luminescence (TSL) by means of the introduction of nanoparticles to improve the sensitivity and selectivity of the analytical methods. TSL detection is usually based on the complexation between fluorescent organic compounds (the analytes) and terbium. The organic compound is then excited, and, after an energy transfer towards terbium, the latter emits the luminescence signal. Here, the modification consists of the introduction of nanoparticles (carbon quantum dots, CQDs) into the system. The carboxylic groups of CQDs react with terbium, providing an interesting time-resolved luminescence probe. We applied this system for the determination of the neonicotinoid imidacloprid (IMID). When IMID was introduced in the terbium-CQDs system, the luminescent signal (λexc/λem of 256/545 nm) was quenched, proportionally to IMID concentration in the range of 100-2500 ng·mL-1, obtaining a limit of detection of 30 ng·mL-1. A method detection limit of 0.9 mg·kg-1 was reached in caneberries, thus complying with the maximum residue level of 5 mg·kg-1 established by Codex Alimentarius. We performed recovery experiments in caneberries (blackberries, blueberries, raspberries, and mulberries), obtaining recovery yields close to 100% in all cases. These results show that the use of terbium ions-nanoparticles luminescence probes can be useful for screening purposes in quality control laboratories.

Bright and Stable Nanomaterials for Imaging and Sensing

This review covers strategies to prepare high-performance emissive polymer nanomaterials, combining very high brightness and photostability, to respond to the drive for better imaging quality and lower detection limits in fluorescence imaging and sensing applications. The more common approaches to obtaining high-brightness nanomaterials consist of designing polymer nanomaterials carrying a large number of fluorescent dyes, either by attaching the dyes to individual polymer chains or by encapsulating the dyes in nanoparticles. In both cases, the dyes can be covalently linked to the polymer during polymerization (by using monomers functionalized with fluorescent groups), or they can be incorporated post-synthesis, using polymers with reactive groups, or encapsulating the unmodified dyes. Silica nanoparticles in particular, obtained by the condensation polymerization of silicon alcoxides, provide highly crosslinked environments that protect the dyes from photodegradation and offer excellent chemical modification flexibility. An alternative and less explored strategy is to increase the brightness of each individual dye. This can be achieved by using nanostructures that couple dyes to plasmonic nanoparticles so that the plasmon resonance can act as an electromagnetic field concentrator to increase the dye excitation efficiency and/or interact with the dye to increase its emission quantum yield.