Dual Amplification Fluorescence Assay for Alpha Fetal Protein Utilizing Immunohybridization Chain Reaction and Metal-Enhanced Fluorescence of Carbon Nanodots

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.

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.

Aptameric Fluorescent Biosensors for Liver Cancer Diagnosis

Liver cancer is a prevalent global health concern with a poor 5-year survival rate upon diagnosis. Current diagnostic techniques using the combination of ultrasound, CT scans, MRI, and biopsy have the limitation of detecting detectable liver cancer when the tumor has already progressed to a certain size, often leading to late-stage diagnoses and grim clinical treatment outcomes. To this end, there has been tremendous interest in developing highly sensitive and selective biosensors to analyze related cancer biomarkers in the early stage diagnosis and prescribe appropriate treatment options. Among the various approaches, aptamers are an ideal recognition element as they can specifically bind to target molecules with high affinity. Furthermore, using aptamers, in conjunction with fluorescent moieties, enables the development of highly sensitive biosensors by taking full advantage of structural and functional flexibility. This review will provide a summary and detailed discussion on recent aptamer-based fluorescence biosensors for liver cancer diagnosis. Specifically, the review focuses on two promising detection strategies: (i) Förster resonance energy transfer (FRET) and (ii) metal-enhanced fluorescence for detecting and characterizing protein and miRNA cancer biomarkers.

Wavelength-Dependent Metal-Enhanced Fluorescence Biosensors via Resonance Energy Transfer Modulation

Fluorescence can be enhanced or quenched depending on the distance between the surface of a metal nanoparticle and the fluorophore molecule. Fluorescence enhancement by nearby metal particles is called metal-enhanced fluorescence (MEF). MEF shows promising potential in the field of fluorescence-based biological sensing. MEF-based biosensor systems generally fall into two platform categories: (1) a two/three-dimensional scaffold, or (2) a colloidal suspension. This review briefly summarizes the application studies using wavelength-dependent carbon dots (UV-VIS), noble metals (VIS), and upconversion nanoparticles (NIR to VIS), representative nanomaterials that contribute to the enhancement of fluorescence through the resonance energy transfer modulation and then presents a perspective on this topic.

Recent advances in plasmon-enhanced luminescence for biosensing and bioimaging

Plasmon-enhanced luminescence (PEL) is a unique photophysical phenomenon in which the interaction between luminescent moieties and metal nanostructures results in a marked luminescence enhancement. PEL offers several advantages and has been extensively used to design robust biosensing platforms for luminescence-based detection and diagnostics applications, as well as for the development of many efficient bioimaging platforms, enabling high-contrast non-invasive real-time optical imaging of biological tissues, cells, and organelles with high spatial and temporal resolution. This review summarizes recent progress in the development of various PEL-based biosensors and bioimaging platforms for diverse biological and biomedical applications. Specifically, we comprehensively assessed rationally designed PEL-based biosensors that can efficiently detect biomarkers (proteins and nucleic acids) in point-of-care tests, highlighting significant improvements in the sensing performance upon the integration of PEL. In addition to discussing the merits and demerits of recently developed PEL-based biosensors on substrates or in solutions, we include a brief discussion on integrating PEL-based biosensing platforms into microfluidic devices as a promising multi-responsive detection method. The review also presents comprehensive details about the recent advances in the development of various PEL-based multi-functional (passive targeting, active targeting, and stimuli-responsive) bioimaging probes, highlighting the scope of future improvements in devising robust PEL-based nanosystems to achieve more effective diagnostic and therapeutic insights by enabling imaging-guided therapy.

Asymmetric parameter enhancement in the split-ring cavity array for virus-like particle sensing

Quantitative detection of virus-like particles under a low concentration is of vital importance for early infection diagnosis and water pollution analysis. In this paper, a novel virus detection method is proposed using indirect polarization parametric imaging method combined with a plasmonic split-ring nanocavity array coated with an Au film and a quantitative algorithm is implemented based on the extended Laplace operator. The attachment of viruses to the split-ring cavity breaks the structural symmetry, and such asymmetry can be enhanced by depositing a thin gold film on the sample, which allows an asymmetrical plasmon mode with a large shift of resonance peak generated under transverse polarization. Correspondingly, the far-field scattering state distribution encoded by the attached virus exhibits a specific asymmetric pattern that is highly correlated to the structural feature of the virus. By utilizing the parametric image sinδ to collect information on the spatial photon state distribution and far-field asymmetry with a sub-wavelength resolution, the appearance of viruses can be detected. To further reduce the background noise and enhance the asymmetric signals, an extended Laplace operator method which divides the detection area into topological units and then calculates the asymmetric parameter is applied, enabling easier determination of virus appearance. Experimental results show that the developed method can provide a detection limit as low as 56 vp/150µL on a large scale, which has great potential in early virus screening and other applications.

Fluorescence immunosensor based on functional nanomaterials and its application in tumor biomarker detection

An immunosensor is defined as an analytical device that detects the binding of an antigen to its specific antibody by coupling an immunochemical reaction to the surface of a device called a transducer. Fluorescence immunosensing is one of the most promising immunoassays at present, and has the advantages of simple operation, fast response and high stability. A traditional fluorescence immunosensor often uses an enzyme-labelled antibody as a recognition unit and an organic dye as a fluorescence probe, so it is easily affected by environmental factors with low sensitivity. Nanomaterials have unique photostability, catalytic properties and biocompatibility, which open up a new path for the construction of stable and sensitive fluorescence immunosensors. This paper briefly introduces different kinds of immunosensors and the role of nanomaterials in the construction of immunosensors. The significance of fluorescent immunosensors constructed from functional nanomaterials to detect tumor biomarkers was analyzed, and the strategies to further improve the performance of fluorescent immunosensors and their future development trend were summarized.

Photothermal-Induced Electrochemical Interfacial Region Regulation Enables Signal Amplification for Dual-Mode Detection of Ovarian Cancer Biomarkers

Detection sensitivity of an electrochemical immunosensor mainly depends on the accessible distance toward the sensing interface; regulating the electrochemical interfacial region thereon is an effective strategy for signal amplification. Herein, a photothermal-regulated sensing interface was designed based on a near-infrared (NIR)-responsive hydrogel probe for ultrasensitive detection of human epididymis protein 4 (HE4). Silver nanoparticle-deposited graphene oxide nanosheet (AgNPs@GO) hybrids as electrochemical signal tags and a photothermal transducer, which were encapsulated in the poly(N-isopropylacrylamide) (pNIPAM) hydrogel, were used to develop the NIR-responsive GO@AgNPs-pNIPAM hydrogel probe. Under NIR light irradiation, the excellent photothermal effect of AgNPs@GO hybrids not only rapidly converted NIR light into heat for temperature readout but also triggered the shrinkage behavior of the hydrogel for electrochemical signal amplification. Compared with the conventional sandwich immunoassay, the shrinkage behavior of the hydrogel signal probe endowed itself with interface regulation capability to improve the electrochemical reaction efficiency; on the basis of ensuring the extended outer Helmholtz plane (OHP) region, the proposed photothermal-induced interface regulation also shortened the OHP, leading to higher sensitivity. Moreover, the obtained dual-mode signals provided satisfactory accuracy for the detection of tumor markers. Therefore, this detection scheme provided an opportunity for the broad applications of the photothermal effect in clinical diagnosis.

Nanomaterial-Based Fluorescence Resonance Energy Transfer (FRET) and Metal-Enhanced Fluorescence (MEF) to Detect Nucleic Acid in Cancer Diagnosis

Nucleic acids, including DNA and RNA, have received prodigious attention as potential biomarkers for precise and early diagnosis of cancers. However, due to their small quantity and instability in body fluids, precise and sensitive detection is highly important. Taking advantage of the ease-to-functionality and plasmonic effect of nanomaterials, fluorescence resonance energy transfer (FRET) and metal-enhanced fluorescence (MEF)-based biosensors have been developed for accurate and sensitive quantitation of cancer-related nucleic acids. This review summarizes the recent strategies and advances in recently developed nanomaterial-based FRET and MEF for biosensors for the detection of nucleic acids in cancer diagnosis. Challenges and opportunities in this field are also discussed. We anticipate that the FRET and MEF-based biosensors discussed in this review will provide valuable information for the sensitive detection of nucleic acids and early diagnosis of cancers.

Substrate-Induced Growth of Micro/Nanostructured Zn(OH)F Arrays for Highly Sensitive Microfluidic Fluorescence Assays

To date, ZnO array-based microfluidic fluorescence assays have been widely investigated and have exhibited excellent performance in the detection of cancer biomarkers. However, the requirements of highly sensitive detection necessitate further improvement of current Zn-based fluorescence detection devices. Here, a rhombus-like Zn(OH)F array-based microfluidic fluorescence detection device is proposed. Construction of Zn(OH)F arrays on the inner wall of a microchannel is carried out via a microfluidic chemical method. A substrate-induced growth strategy for Zn(OH)F arrays is proposed, and various micro/nanostructured Zn(OH)F arrays are successfully obtained. Zn(OH)F nanorod arrays with a high aspect ratio can be constructed on the columnar ZnO nanorod arrays, and the results indicate that the fluorescence enhancement factor (EF) of the Zn(OH)F arrays toward Cy3 is approximately 4-fold that of the ZnO nanorod arrays, which can be attributed to the higher excitation light absorption and evanescent electric field. In human epididymis-specific protein 4 (HE4) detection, the limit of detection (LOD) reaches 9.3 fM, and the dynamic linear range is 10 fM to 100 pM. It has been demonstrated that Zn(OH)F nanorod array-based microfluidic devices are excellent fluorescence assay platforms that also provide a new design and construction strategy for fluorescence enhancement substrates for the detection of biomarkers.

Recent progress in sensing application of metal nanoarchitecture-enhanced fluorescence

Fluorescence analytical methods, as real time and in situ analytical approaches to target analytes, can offer advantages of high sensitivity/selectivity, great versatility, non-invasive measurement and easy transmission over long distances. However, the conventional fluorescence assay still suffers from low specificity, insufficient sensitivity, poor reliability and false-positive responses. By exploiting various metal nanoarchitectures to manipulate fluorescence, both increased fluorescence quantum yield and improved photostability can be realized. This metal nanoarchitecture-enhanced fluorescence (MEF) phenomenon has been extensively studied and used in various sensors over the past years, which greatly improved their sensing performance. Thus in this review, we primarily give a general overview of MEF based sensors from mechanisms to state-of-the-art applications in environmental assays, biological/medical analysis and diagnosis areas. Finally, their pros and cons as well as further development directions are also discussed.

A colorimetric and electrochemical dual-mode biosensor for thrombin using a magnetic separation technique

In general, protein detection relies primarily on enzyme-linked immunosorbent assays. Here, we constructed a colorimetric and electrochemical dual-mode biosensor for thrombin detection based on the mechanism of aptamer recognition. Magnetic nanobeads (MBs) were used as carriers for separation and enrichment to quickly capture thrombin (TB) in the complex matrix. Also, the combination of MBs and the magnetic electrode array (MEA) effectively avoided the poisoning of the electrode by biological samples. Furthermore, hybridization chain reaction (HCR) was indirectly used to achieve amplification of TB. A large number of horseradish peroxidases (HRPs) were coupled with the amplified long nucleic acid fragments. Based on the color and current response of the substrate TMB catalyzed by HRP, a dual-mode detection system for thrombin was established to ensure the accuracy of the test results. The method had a minimum resolution of 10 nM to the naked eye and an electrochemical detection limit as low as 0.35 nM. In addition, the sensor provided good anti-interference ability in a complex matrix and showed great potential to detect TB in complex samples.

A simple electrochemical immunosensor based on worm-like platinum for highly sensitive determination of alpha-fetoprotein

Alpha-fetoprotein (AFP) has become a specific tumor marker of primary liver cancer in clinical diagnosis. In this work, we prepared worm-like platinum (WL Pt) nanomaterial via chemical etching without organic solvents and ultra-high temperature. Due to its small particle size and the formation of surface vacancies during the etching process, it had a large specific surface area, and thus exhibited superior electrocatalytic activity for the reduction of hydrogen peroxide. Combining the signal amplification based on hydrogen peroxide reduction and the specific recognition of antigen with antibody, we constructed a simple label-free electrochemical immunosensor with a sandwich-like structure. The developed electrochemical immunosensor showed a wide linear range (0.0001-100 ng mL^-1), a low detection limit (0.028 pg mL^-1), good selectivity and stability. Further, the immunosensor was comparable with enzyme-linked immunosorbent assay (ELISA) and had a good accuracy for AFP detection in human serum samples proving the feasibility of potential application, which is expect to become one of the most promising method in early diagnosis and clinical analysis for liver cancer.

Surface chemistry tuning the selectivity of carbon nanodots towards Hg2+ recognition

Fluorescence quenching of carbon nanodots by metal ions has been extensively applied for the determination of oligonucleotides, proteins, small molecules and metal ions. However, the problem of poor selectivity originating from the coordination of surface oxygen-containing groups to many kinds of metal ions has limited the prosperity of carbon nanodots in detection field. Herein, the specific recognition of carbon nanodots to Hg²⁺ is controlled by rational regulation of the surface structure of carbon nanodots. Passivation of the surface carboxyl and hydroxyl groups plays a decisive role in inhibiting the binding of metal ions with carbon nanodots. Upon the attachment of Hg²⁺ specific recognition unit, carbon nanodots exhibited a high selectivity to Hg²⁺. This work facilitates to rationally design the surface structure of carbon nanodots to obtain the desirable selective recognition ability.

Review: Applications of surface-enhanced fluorescence (SEF) spectroscopy in bio-detection and biosensing

Surface-enhanced fluorescence (SEF) is rapidly becoming one of the main spectroscopic techniques for the detection of a variety of biomolecules and biomarkers. The main reasons for this trend are the high sensitivity and selectivity, robustness, and speed of this analytical method. Each year, the number of applications that utilize this phenomenon increases and with each such work, the complexity and novelty of the used substrates, procedures, and analytes rises. To obtain a clearer view of this phenomenon and research area, we decided to combine 76 valuable research articles from a variety of different research groups into this mini-review. We present and describe these works concisely and clearly, with a particular interest in the quantitative parameters of the experiment. These sources are classified according to the nature of the analyte, on the contrary to most reviews, which sort them by substrate nature. This point of view gives us insight into the development of this research area and the consequent increase in the complexity of the analyte nature. Moreover, this type of sorting can show possible future routes for the expansion of this research area. Along with the analytes, we can also pay attention to the substrates used for each situation and how the development of substrates affects the direction of research and subsequently, the choice of an analyte. About 108 sources and several interesting trends in the SEF research area over the past 25 years are discussed in this mini-review.

Recent Developments of Carbon Dots in Biosensing: A Review

Early diagnosis of diseases is of great importance because it increases the chance of a cure and significantly reduces treatment costs. Thus, development of rapid, sensitive, and reliable biosensing techniques is essential for the benefits of human life and health. As such, various nanomaterials have been explored to improve performance of biosensors, among which, carbon dots (CDs) have received enormous attention due to their excellent performance. In this Review, the recent advancements of CD-based biosensors have been carefully summarized. First, biosensors are classified according to their sensing strategies, and the role of CDs in these sensors is elaborated in detail. Next, several typical CD-based biosensors (including CD-only, enzymatic, antigen-antibody, and nucleic acid biosensors) and their applications are fully discussed. Last, advantages, challenges, and perspectives on the future trends of CD-based biosensors are highlighted.

A universal assay strategy for sensitive and simultaneous quantitation of multiplex tumor markers based on the stirring rod-immobilized DNA-LaMnO3 perovskite-metal ions encoded probes

It was extremely urgent to develop some simultaneous and sensitive biosensors for detecting multiplex serum tumor markers (TMs) for early screening of cancers. Herein, a multiplex assay was developed based on the DNA-LaMnO₃ (DNA-LMO) perovskite encoded probes and targets mediated competitive replacement strategy. Alpha fetoprotein (AFP), carcinoembryonic antigen (CEA) and prostate specific antigen (PSA) markers were employed as representative target TMs. Aptasensor is prepared by a series of DNA-LMO-M encode probes which were prepared by three hyperbranched DNA firstly immobilized on LMO encapsulating Pb, Cd or Cu ions. Then, three TMs aptamers were labeled on the stirring-rod and hybridized with the probes. After the developed encoded probes was incubated the TMs, the encoded probes corresponding to different TMs can be released into the supernatant through the competitive replacement. The inner metal ion can be simultaneously detected by square wave voltammetry corresponding to various TMs. Since the stirring rod can enrich many encoded probes containing a lot of metal ions, multiplex signal amplification can be realized. Due to the enrichment and easy separation of the stirring rod, the signal-to-noise ratio was also obviously improved and thus to results in good sensitivity and accuracy. Moreover, it took only 20 min to detect three targets which much faster than many same types of aptasensor. Under the optimal conditions, the low detection limit for CEA (3.6 × 10^-4 ng/mL), AFP (3.4 × 10^-4 ng/mL) and PSA (2.8 × 10^-4 ng/mL) were obtained. Therefore, this method is likely to be used for early and sensitive screening of tumors.

Plasmon-Emitter Hybrid Nanostructures of Gold Nanorod-Quantum Dots with Regulated Energy Transfer as a Universal Nano-Sensor for One-step Biomarker Detection

Recently, biosensing based on weak coupling in plasmon-emitter hybrid nanostructures exhibits the merits of simplicity and high sensitivity, and attracts increasing attention as an emerging nano-sensor. In this study, we propose an innovative plasmon-regulated fluorescence resonance energy transfer (plasmon-regulated FRET) sensing strategy based on a plasmon-emitter hybrid nanostructure of gold nanorod-quantum dots (Au NR-QDs) by partially modifying QDs onto the surfaces of Au NRs. The Au NR-QDs showed good sensitivity and reversibility against refractive index change. We successfully employed the Au NR-QDs to fabricate nano-sensors for detecting a cancer biomarker of alpha fetoprotein with a limit of detection of 0.30 ng/mL, which displays that the sensitivity of the Au NR-QDs nano-sensor was effectively improved compared with the Au NRs based plasmonic sensing. Additionally, to demonstrate the universality of the plasmon-regulated FRET sensing strategy, another plasmon-emitter hybrid nano-sensor of Au nano-prism-quantum dots (Au NP-QDs) were constructed and applied for detecting a myocardial infarction biomarker of cardiac troponin I. It was first reported that the change of absorption spectra of plasmonic structure in a plasmon-emitter hybrid nanostructure was employed for analytes detection. The plasmon-regulated FRET sensing strategy described herein has potential utility to develop general sensing platforms for chemical and biological analysis.

A boronic acid carbon nanodots/poly(thionine) sensing platform for the accurate and reliable detection of NADH

In this work, a novel electrochemical sensing platform was designed and fabricated by the modification of boronic acid functionalized carbon nanodots (B-CNDs) and poly(thionine) (pTHI) on an electrode surface. B-CNDs can not only accelerate electron transfer but also covalently interact with cis-diol groups of dihydronicotinamide adenine dinucleotide (NADH) through functionalized boronic acid groups. Meanwhile, pTHI served as an inner reference element to provide a built-in correction, which enabled the sensor to detect NADH with high accuracy and reliability based on a ratiometric signal (∆I_NADH/∆I_THI). The electrochemical experimental results demonstrated that the ratiometric strategy-based sensor possessed good selectivity and high sensitivity. A linear range of 5.0 × 10^-7 - 2.0 × 10^-4 mol/L for NADH detection was obtained with a limit of detection of 1.5 × 10^-7 mol/L. The sensor has been applied to analyze NADH in human serum samples with satisfactory results. The simple and effective ratiometric strategy reported here can be further used to prepare electrochemical sensors for selective, sensitive, and reliable detection of other cis-diol compounds.

Plasmon-Enhanced Fluorescence of Carbon Nanodots in Gold Nanoslit Cavities

Carbon nanodots (CNDs) are featured with a wide range of light absorption and excitation-dependent fluorescence. The emission enhancement of CNDs is of great interest for the development of nanophotonics. Although the phenomenon of plasmon-enhanced fluorescence for quantum dots and molecular dyes has been well investigated, rarely has it been reported for CNDs. In this work, a series of plasmonic nanoslit designs were fabricated and utilized for immobilization of CNDs in nanoslits and examination of the best match for plasmonic fluorescence enhancement of CNDs. In concert, to better understand the plasmonic effect on the enhancement, the surface optical field is measured with or without CND immobilization using a hyperspectral imaging system as a comparison, and a semianalytical model is conducted for a quantitative analysis of surface plasmon generation under the plane-wave illumination. Both the fluorescence and surface reflection light intensity enhancement are demonstrated as a function of nanoslit width and are maximized at the 100 nm nanoslit width. The analysis of surface plasmon-exciton coupling of CNDs in the nanoslit area suggests that the enhancement is primarily due to plasmonic light trapping for increased electromagnetic field and plasmon-induced resonance energy transfer. This study suggests that incorporating CNDs in the plasmonic nanoslits may provide a largely enhanced CND-based photoemission system for optical applications.

Ultrasensitive detection of heparin by exploiting the silver nanoparticle-enhanced fluorescence of graphitic carbon nitride (g-C3N4) quantum dots

A composite (Ag-g-CNQDs) was prepared from graphitic carbon nitride quantum dots and silver nanoparticles by water phase synthesis. Aided by metal-enhanced fluorescence, the composite exhibits excitation-dependent red emission with a peak at 600 nm with a quantum yield of 21%. If the composite is coated with polyethylenimine (PEI) to form the Ag-g-CNQD/PEI complexe, fluorescence is strongly reduced. Upon addition of heparin, the fluorescence of the system is enhanced because PEI has a higher affinity for heparin than Ag-g-CNQDs. The effect was used to design a fluorometric  assay for heparin. The emission at 600 nm increases linearly in the 0.025 to 2.5 μM heparin concentration range, with a 8.2 nM limit of detection. Graphical abstract Schematic illustration for fabricating a composite consisting of silver nanoparticles and graphitic carbon nitride quantum dots (Ag-g-CNQDs). Its red fluorescence is weak in presence of polyethyleneimine but restored on addition of heparin. This forms the basis for a new method for heparin detection.