Ultrastable Plasmonic Bioink for Printable Point-Of-Care Biosensors

Surface-Enhanced Raman Spectroscopy for Biomedical Applications: Recent Advances and Future Challenges

The year 2024 marks the 50th anniversary of the discovery of surface-enhanced Raman spectroscopy (SERS). Over recent years, SERS has experienced rapid development and became a critical tool in biomedicine with its unparalleled sensitivity and molecular specificity. This review summarizes the advancements and challenges in SERS substrates, nanotags, instrumentation, and spectral analysis for biomedical applications. We highlight the key developments in colloidal and solid SERS substrates, with an emphasis on surface chemistry, hotspot design, and 3D hydrogel plasmonic architectures. Additionally, we introduce recent innovations in SERS nanotags, including those with interior gaps, orthogonal Raman reporters, and near-infrared-II-responsive properties, along with biomimetic coatings. Emerging technologies such as optical tweezers, plasmonic nanopores, and wearable sensors have expanded SERS capabilities for single-cell and single-molecule analysis. Advances in spectral analysis, including signal digitalization, denoising, and deep learning algorithms, have improved the quantification of complex biological data. Finally, this review discusses SERS biomedical applications in nucleic acid detection, protein characterization, metabolite analysis, single-cell monitoring, and in vivo deep Raman spectroscopy, emphasizing its potential for liquid biopsy, metabolic phenotyping, and extracellular vesicle diagnostics. The review concludes with a perspective on clinical translation of SERS, addressing commercialization potentials and the challenges in deep tissue in vivo sensing and imaging.

Advancements in 3D skin bioprinting: processes, bioinks, applications and sensor integration

This comprehensive review explores the multifaceted landscape of skin bioprinting, revolutionizing dermatological research. The applications of skin bioprinting utilizing techniques like extrusion-, droplet-, laser- and light-based methods, with specialized bioinks for skin biofabrication have been critically reviewed along with the intricate aspects of bioprinting hair follicles, sweat glands, and achieving skin pigmentation. Challenges remain with the need for vascularization, safety concerns, and the integration of automated processes for effective clinical translation. The review further investigates the incorporation of biosensor technologies, emphasizing their role in monitoring and enhancing the wound healing process. While highlighting the remarkable progress in the field, critical limitations and concerns are critically examined to provide a balanced perspective. This synthesis aims to guide scientists, engineers, and healthcare providers, fostering a deeper understanding of the current state, challenges, and future directions in skin bioprinting for transformative applications in tissue engineering and regenerative medicine.

Self-Adhesive, Biocompatible, Wearable Microfluidics with Erasable Liquid Metal Plasmonic Hotspots for Glucose Detection in Sweat

Sweat is a noninvasive metabolite that can provide clinically meaningful information about physical conditions without harming the body. Glucose, a vital component in sweat, is closely related to blood glucose levels, and changes in its concentration can reflect the health status of diabetics. We introduce a self-adhesive, wearable microfluidic chip with erasable liquid metal plasmonic hotspots for the precise detection of glucose concentration in sweat. The self-adhesive, wearable microfluidic chip is made from modified polydimethylsiloxane (PDMS) with enhanced stickiness, enabling conformal contact with the skin, and can collect, deliver, and store sweat. The plasmonic hotspots are located inside the microfluidic channel, are generated by synthesizing silver nanostructures on liquid metal, and can be removed in the alkaline solution. It indicates the erasable and reproducible nature of the plasmonic hotspots. The detection method is based on surface-enhanced Raman spectroscopy (SERS), which allows for accurate detection of the glucose concentration. To enhance the sensitive detection of glucose, the SERS substrate is modified by 4-mercaptophenylboronic acid to achieve the limit of detection of 1 ng/L glucose, which is much lower than the physiological conditions (7.2-25.2 μg/L). The developed microfluidic chip is soft, stretchable, and nontoxic, bringing new possibilities to wearable sweat-sensing devices.

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.

Bioinks and biofabrication techniques for biosensors development: A review

3D bioprinting technologies and bioink development are enabling significant advances in miniaturized and integrated biosensors. For example, bioreceptors can be immobilized within a porous 3D structure to significantly amplify the signal, while biocompatible and mechanically flexible systems uniquely enable wearable chem- and bio-sensors. This advancement is accelerating translation by enabling the production of high performance, reproducible, and flexible analytical devices. The formulation of the bioink plays a crucial role in determining the bio-functionality of the resulting printed structures, e.g., the porosity that allows the analyte to diffuse through the 3D structure, the affinity and avidity of the receptors, etc. This review explores the next generation of advanced bioinks for biosensor development and provides insights into the latest cutting-edge bioprinting technologies. The bioprinting methods available for biosensor fabrication including inkjet, extrusion, and laser-based bioprinting, are discussed. The advantages and limitations of each method are analysed, and recent advancements in bioprinting technologies are presented. The review then delves into the properties of advanced bioinks, such as biocompatibility, printability, stability, and applicability. Different types of advanced bioinks are explored, including multicomponent, stimuli-responsive, and conductive bioinks. Finally, the next generation of bioinks for biosensors is considered, identifying possible new opportunities and challenges. Overall, this literature review highlights the combined importance of bioink formulation and bioprinting methods for the development of high-performance analytical biosensors.

Molecularly Imprinted Wearable Sensor with Paper Microfluidics for Real-Time Sweat Biomarker Analysis

The urgent need for real-time and noninvasive monitoring of health-associated biochemical parameters has motivated the development of wearable sweat sensors. Existing electrochemical sensors show promise in real-time analysis of various chemical biomarkers. These sensors often rely on labels and redox probes to generate and amplify the signals for the detection and quantification of analytes with limited sensitivity. In this study, we introduce a molecularly imprinted polymer (MIP)-based biochemical sensor to quantify a molecular biomarker in sweat using electrochemical impedance spectroscopy, which eliminates the need for labels or redox probes. The molecularly imprinted biosensor can achieve sensitive and specific detection of cortisol at concentrations as low as 1 pM, 1000-fold lower than previously reported MIP cortisol sensors. We integrated multimodal electrochemical sensors with an iontophoresis sweat extraction module and paper microfluidics for real-time sweat analysis. Several parameters can be simultaneously quantified, including sweat volume, secretion rate, sodium ion, and cortisol concentration. Paper microfluidic modules not only quantify sweat volume and secretion rate but also facilitate continuous sweat analysis without user intervention. While we focus on cortisol sensing as a proof-of-concept, the molecularly imprinted wearable sensors can be extended to real-time detection of other biochemicals, such as protein biomarkers and therapeutic drugs.

Nanomaterial surface modification toolkit: Principles, components, recipes, and applications

Surface-functionalized nanostructures are at the forefront of biotechnology, providing new opportunities for biosensors, drug delivery, therapy, and bioimaging applications. The modification of nanostructures significantly impacts the performance and success of various applications by enabling selective and precise targeting. This review elucidates widely practiced surface modification strategies, including click chemistry, cross-coupling, silanization, aldehyde linkers, active ester chemistry, maleimide chemistry, epoxy linkers, and other protein and DNA-based methodologies. We also delve into the application-focused landscape of the nano-bio interface, emphasizing four key domains: therapeutics, biosensing, environmental monitoring, and point-of-care technologies, by highlighting prominent studies. The insights presented herein pave the way for further innovations at the intersection of nanotechnology and biotechnology, providing a useful handbook for beginners and professionals. The review draws on various sources, including the latest research articles (2018-2023), to provide a comprehensive overview of the field.

Ultrasensitive, Multiplexed Buoyant Sensor for Monitoring Cytokines in Biofluids

Multiplexed quantification of low-abundance protein biomarkers in complex biofluids is important for biomedical research and clinical diagnostics. However, in situ sampling without perturbing biological systems remains challenging. In this work, we report a buoyant biosensor that enables in situ monitoring of protein analytes at attomolar concentrations with a 15 min temporal resolution. The buoyant biosensor implemented with fluorescent nanolabels enabled the ultrasensitive and multiplexed detection and quantification of cytokines. Implementing the biosensor in a digital manner (i.e., counting the individual nanolabels) further improves the low detection limit. We demonstrate that the biosensor enables the detection and quantification of the time-varying concentrations of cytokines (e.g., IL-6 and TNF-α) in macrophage culture media without perturbing the live cells. The easy-to-apply biosensor with attomolar sensitivity and multiplexing capability can enable an in situ analysis of protein biomarkers in various biofluids and tissues to aid in understanding biological processes and diagnosing and treating diverse diseases.

A label-free electrochemical aptasensor based on Bi-Sb alloy materials for potential POCT of HER-2

As a prognostic biomarker for breast cancer, human epidermal growth factor receptor 2 (HER-2) is of crucial diagnostic value. Here, a label-free electrochemical aptasensor was established for the ultrasensitive detection of HER-2 using a modified electrode of Bi-Sb alloy materials (Bi-Sb AMs). The performance of the aptasensor was enhanced greatly due to the introduction of Bi-Sb alloy materials (Bi-Sb AMs) with high conductivity. Furthermore, by integrating the aptasensor with the Sensit Smart U-disk electrochemical analyzer, the point-of-care testing (POCT) for HER-2 was realized. Under the optimal experimental parameters, the POCT analyzer showed a wide linear response from 0.01 pg mL-1 to 100 ng mL-1, with a low detection limit (LOD) of 5.96 fg mL-1 for the detection of HER-2. The presented POCT analyzer exhibited good specificity, stability, and reproducibility. Benefiting from the simple operation and rapid testing, the developed analyzer will have potential application in the prognostic diagnosis and treatment of breast cancer.

Low-dose albumin-coated gold nanorods induce intercellular gaps on vascular endothelium by causing the contraction of cytoskeletal actin

Cytotoxicity of nanoparticles, typically evaluated by biochemical-based assays, often overlook the cellular biophysical properties such as cell morphology and cytoskeletal actin, which could serve as more sensitive indicators for cytotoxicity. Here, we demonstrate that low-dose albumin-coated gold nanorods (HSA@AuNRs), although being considered noncytotoxic in multiple biochemical assays, can induce intercellular gaps and enhance the paracellular permeability between human aortic endothelial cells (HAECs). The formation of intercellular gaps can be attributed to the changed cell morphology and cytoskeletal actin structures, as validated at the monolayer and single cell levels using fluorescence staining, atomic force microscopy, and super-resolution imaging. Molecular mechanistic study shows the caveolae-mediated endocytosis of HSA@AuNRs induces the calcium influx and activates actomyosin contraction in HAECs. Considering the important roles of endothelial integrity/dysfunction in various physiological/pathological conditions, this work suggests a potential adverse effect of albumin-coated gold nanorods on the cardiovascular system. On the other hand, this work also offers a feasible way to modulate the endothelial permeability, thus promoting drug and nanoparticle delivery across the endothelium.

Traction of 3D and 4D Printing in the Healthcare Industry: From Drug Delivery and Analysis to Regenerative Medicine

Three-dimensional (3D) printing and 3D bioprinting are promising technologies for a broad range of healthcare applications from frontier regenerative medicine and tissue engineering therapies to pharmaceutical advancements yet must overcome the challenges of biocompatibility and resolution. Through comparison of traditional biofabrication methods with 3D (bio)printing, this review highlights the promise of 3D printing for the production of on-demand, personalized, and complex products that enhance the accessibility, effectiveness, and safety of drug therapies and delivery systems. In addition, this review describes the capacity of 3D bioprinting to fabricate patient-specific tissues and living cell systems (e.g., vascular networks, organs, muscles, and skeletal systems) as well as its applications in the delivery of cells and genes, microfluidics, and organ-on-chip constructs. This review summarizes how tailoring selected parameters (i.e., accurately selecting the appropriate printing method, materials, and printing parameters based on the desired application and behavior) can better facilitate the development of optimized 3D-printed products and how dynamic 4D-printed strategies (printing materials designed to change with time or stimulus) may be deployed to overcome many of the inherent limitations of conventional 3D-printed technologies. Comprehensive insights into a critical perspective of the future of 4D bioprinting, crucial requirements for 4D printing including the programmability of a material, multimaterial printing methods, and precise designs for meticulous transformations or even clinical applications are also given.

Wearable plasmonic paper-based microfluidics for continuous sweat analysis

Wearable sweat sensors have the potential to provide clinically meaningful information associated with the health and disease states of individuals. Current sensors mainly rely on enzymes and antibodies as biorecognition elements to achieve specific quantification of metabolite and stress biomarkers in sweat. However, enzymes and antibodies are prone to degrade over time, compromising the sensor performance. Here, we introduce a wearable plasmonic paper-based microfluidic system for continuous and simultaneous quantitative analysis of sweat loss, sweat rate, and metabolites in sweat. Plasmonic sensors based on label-free surface-enhanced Raman spectroscopy (SERS) can provide chemical "fingerprint" information for analyte identification. We demonstrate the sensitive detection and quantification of uric acid in sweat at physiological and pathological concentrations. The well-defined flow characteristics of paper microfluidic devices enable accurate quantification of sweat loss and sweat rate. The wearable plasmonic device is soft, flexible, and stretchable, which can robustly interface with the skin without inducing chemical or physical irritation.

Printed Ultrastable Bioplasmonic Microarrays for Point-of-Need Biosensing

Paper-based point-of-need (PON) biosensors are attractive for various applications, including food safety, agriculture, disease diagnosis, and drug screening, owing to their low cost and ease of use. However, existing paper-based biosensors mainly rely on biolabels, colorimetric reagents, and biorecognition elements and exhibit limited stability under harsh environments. Here, we report a label-free paper-based biosensor composed of bioplasmonic microarrays for sensitive detection and quantification of protein targets in small volumes of biofluids. Bioplasmonic microarrays were printed using an ultrastable bioplasmonic ink, rendering the PON sensors excellent thermal, chemical, and biological stability for their reliable performance in resource-limited settings. We fabricated silicone hydrophobic barriers and bioplasmonic microarrays with direct writing and droplet jetting approaches on a three-dimensional (3D) nanoporous paper. Direct writing hydrophobic barriers can define hydrophilic channels less than 100 μm wide. High-resolution patterning of hydrophilic test domains enables the handling and analysis of small fluid volumes. We show that the plasmonic sensors based on a vertical flow assay provide similar sensitivity and low limit of detection with a 60 μL sample volume compared to those with 500 μL samples based on an immersion approach and can shorten assay time from 90 to 20 min.

A compact photonic resonator absorption microscope for point of care digital resolution nucleic acid molecular diagnostics

Rapid, sensitive, and selective detection of nucleic acid biomarkers for health diagnostic applications becomes feasible for point of care scenarios when the detection instrument is inexpensive, simple, and robust. Here, we report the design, implementation, and characterization of a point of care instrument for photonic resonator absorption microscopy (PRAM) that takes advantage of resonant optical coupling between plasmonic gold nanoparticle tags and a photonic crystal (PC) surface. Matching the PC resonant wavelength to the gold nanoparticle's surface plasmon wavelength generates localized and efficient quenching of the PC resonant reflection intensity, resulting in the ability to clearly detect and count individual gold nanoparticles when they are captured on the PC surface. Surface-captured nanoparticles are observed by illuminating the PC at normal incidence with polarized light from a low-intensity red LED, and recording of PC reflected intensity on an inexpensive CMOS image sensor. A contrast limited adaptive histogram equalization (CLAHE) image processing algorithm was applied to derive counts of captured nanoparticles. The instrument is utilized in the context of an activate capture + digital counting (AC + DC) assay for a specific miRNA sequence, using nucleic acid toehold probes applied to gold nano-urchin (AuNU) nanoparticles to achieve 160 aM detection limits in a 30 min. assay.

Bi2S3@MoS2 Nanoflowers on Cellulose Fibers Combined with Octahedral CeO2 for Dual-Mode Microfluidic Paper-Based MiRNA-141 Sensors

An effective dual-mode microfluidic paper-based analysis device (μPAD) was proposed via Bi₂S₃@MoS₂ nanoflowers combined with octahedral CeO₂ for ultrasensitive miRNA-141 bioassay. To obtain the amplified electrochemical signal, Bi₂S₃@MoS₂ nanoflowers were first in situ grown onto the surface of cellulose fibers to promote the reduction of H₂O₂. The prism-anchored octahedral CeO₂ nanoparticles with a great catalytic function on the reduction of H₂O₂ were linked up to the functionalized cellulose fibers through the hybridization chain reaction to further enhance the electrochemical signal. By means of the catalysis effect of Bi₂S₃@MoS₂ nanoflowers and octahedral CeO₂ nanoparticles, the obtained signal was amplified, thereby achieving ultrasensitive electrochemical detection of the target. With the help of duplex specific nuclease, the octahedral CeO₂ could be released from the electrochemical detection area and flow to the color channel through capillary action, which could initiate the oxidation reaction of 3,3',5,5'-tetramethylbenzidine in the existence of H₂O₂ to generate a blue visual band, avoiding the error of distinguishing color depth caused by the naked eye and thus improving the accuracy of the visual method. Under the optimal conditions, satisfactory prediction and accurate detection performance were achieved in the range of 10 fM-1 nM and 0.5 fM-1 nM, respectively, by measuring the length of the blue product and the electrochemical signal intensity. The electrochemical/visual detection limits of the proposed μPAD for miRNA-141 were as low as 0.12 and 2.65 fM (S/N = 3). This work provides great potential for the construction of low-cost and high-performance dual-mode biosensors for the detection of biomarkers.

Detection of the Chilli Leaf Curl Virus Using an Attenuated Total Reflection-Mediated Localized Surface-Plasmon-Resonance-Based Optical Platform

The development of a nanoparticle-based optical platform has been presented as a biosensor for detecting target-specific plant virus DNA. The binding dynamics of gold nanoparticles has been studied on the amine-functionalized surface by the attenuated total reflection (ATR)-based evanescent wave absorption method monitoring the localized surface plasmon resonance (LSPR). The developed surface was established as a refractive index sensor by monitoring the LSPR absorption peak of gold nanoparticles. This nanoparticle-immobilized surface was explored to establish as a biosensing platform with target-specific immunoglobulin (IgG) antibody-antigen interaction. The IgG concentration-dependent variation of absorbance was correlated with the refractive index change. After successfully establishing this ATR configuration as an LSPR-based biosensor, the single-stranded DNA of the chilli leaf curl virus was detected using its complementary DNA sequence as a receptor. The limit of detection of this sensor was determined to be 1.0 μg/mL for this target viral DNA. This ATR absorption technique has enormous potential as an LSPR based nano-biosensor for the detection of other begomoviruses.

Stabilization of surface-bound antibodies for ELISA based on a reversable zeolitic imidazolate framework-8 coating

Immunoassays typically must be stored under refrigerated conditions because antibodies, after being immobilized to solid surfaces, tend to lose their recognition capabilities to target antigens under non-refrigerated conditions. This requirement hinders application of immunoassays in resource-limited settings including rural clinics in tropical regions, disaster struck areas, and low-income countries, where refrigeration may not be feasible. In this work, a facile approach based on a reversable zeolitic imidazolate framework-8 (ZIF-8) coating is introduced to stabilize surface-bound antibodies on enzyme-linked immunosorbent assay (ELISA) plates under non-refrigerated conditions. Using a sandwich ELISA for the detection of neutrophil gelatinase-associated lipocalin (NGAL), a urine biomarker for acute kidney injury, as a model system, ZIF-8 is demonstrated to be able to uniformly coat the surface-bound anti-NGAL IgG, and stabilize the dynamic range and detection sensitivity of the assay after storage at an elevated temperature (50 °C) for at least 4 weeks. The stabilization efficacy of the ZIF-8 coating is comparable to the current "gold standard" refrigeration approach, and superior to the commonly used sucrose coating method. This approach will greatly improve the shelf-life and stability of antibody-coated ELISAs and other types of assays which utilize surface-bound antibodies, thus extending biomedical research and medical diagnostics to resource-limited settings.

Metal-Organic Frameworks for Preserving the Functionality of Plasmonic Nanosensors

Preserving the functionality of nanosensors is critical for their reliable performance under harsh environmental conditions. Biofunctionalized plasmonic nanostructures are an important class of bionanoconjugates for biosensing, bioimaging, and nanotherapeutics. Plasmonic nanostructures and biomolecules exhibit poor thermal stability over time. Here, we report a class of metal-organic framework, zeolitic imidazolate framework-8 (ZIF-8), as a protective coating for preserving plasmonic nanostructures and plasmonic bionanoconjugates at elevated temperature. Gold nanobipyramids (AuNBPs) with sharp tips are attractive plasmonic nanotransducers with high sensitivity but are prone to structural change and loss of sensitivity. This work reports the first observation that ZIF-8 can preserve the structure of AuNBPs and their corresponding strong electromagnetic field enhancement and high refractive index sensitivity. In addition, ZIF-8 coating enables nearly 100% retention of biorecognition capability of antibodies immobilized on the AuNBP surface after exposure to 60 °C for 48 h. The efficacy, versatility, and facile implementation of ZIF-8 coating offer great promise for the preservation of nanosensors.