Gold Nanorod Size-Dependent Fluorescence Enhancement for Ultrasensitive Fluoroimmunoassays

Electrochemiluminescence sensor for organophosphorus pesticides based on the regulation of resonance energy transfer between negative charged gold nanorods and Ru(bpy)32

Combined the electrostatic interaction of the negatively charged gold nanorods (AuNRs) (as acceptor) and Ru(bpy)32+ (as donor), an electrochemiluminescence resonance energy transfer (ECL-RET) sensor was constructed and applied for the detection of organophosphorus pesticides (OPs). Negatively charged AuNRs were synthesized by modifying AuNRs with polystyrene sulfonate (PSS) firstly, which can bind to Ru(bpy)32+ through electrostatic interaction so that the luminophore was absorbed by the acceptor, the resonance energy transfer occurred and only low ECL signal had been detected. Thiocholine can be produced by the hydrolysis process of acetylthiocholine (ATCh) with the help of acetylcholinesterase (AChE), which can bond with PSS-modified AuNRs (PSS-AuNRs) through gold-sulfur interaction, this caused the releasing of the adsorbed Ru(bpy)32+ into the solution and resulting in the restoration of the ECL intensity. However, the activity of AChE was inhibited by OPs, and the recovery process of the ECL signal was thus suppressed as well. In this study, chlorpyrifos was chosen as model target, the results indicated that the correlation between the ECL intensity and the logarithm of chlorpyrifos concentration showed remarkable linearity across 1 ng/mL to 1 mg/mL, achieving a detection limit of 0.51 ng/mL. The proposed system has been utilized for detecting OPs in real samples with satisfied results.

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.

Fabrication of Supported and Unsupported Gold Nanorods for Nonenzymatic Glucose Sensing and Study of Their Growth Kinetics

This work includes a novel approach for synthesis/fabrication of AuNRs of varied aspect ratios leading to investigation on the kinetics of their growth mechanism. The synthesized AuNRs were further functionalized with MWCNTs (AuNRs@MWCNTs) by one-pot synthesis. The synthesized AuNRs and AuNRs@MWCNTs were characterized by employing UV-vis spectroscopy. Red shifts in the spectra of AuNRs confirmed the formation of nanorods of higher aspect ratios. Morphology of AuNRs and functionalized AuNRs was confirmed by high-resolution scanning electron microscopy. Biological studies were carried out by fabricating efficient nonenzymatic glucose sensors for optical and electrochemical sensing via UV and cyclic voltammetry in the detection ranges of 0.7-28 mM glucose (UV) and 5.5 μM-0.33 mM (CV). An electrochemical sensing study was carried out via AuNR- and AuNRs@MWCNT-modified GCEs in a 0.1 M NaOH electrolyte solution. The modified electrodes exhibited very high sensitivity with a broad linear range. The order of sensitivity (via CV) was found to be AuNRX0@MWCNTs > AuNRD5@MWCNTs > AuNRD5 > AuNRX0.

Customized Self-Assembled Gold Nanoparticle-DNA Origami Composite Templates for Shape-Directed Growth of Plasmonic Structures

The metal plasmonic nanostructure has the optical property of plasmon resonance, which holds great potential for development in nanophotonics, bioelectronics, and molecular detection. However, developing a general and straightforward method to prepare metal plasmonic nanostructures with a controllable size and morphology still poses a challenge. Herein, we proposed a synthesis strategy that utilized a customizable self-assembly template for shape-directed growth of metal structures. We employed gold nanoparticles (AuNPs) as connectors and DNA nanotubes as branches, customizing gold nanoparticle-DNA origami composite nanostructures with different branches by adjusting the assembly ratio between the connectors and branches. Subsequently, various morphologies of plasmonic metal nanostructures were created using this template shape guided strategy, which exhibited enhancement of surface-enhanced Raman scattering (SERS) signals. This strategy provides a new approach for synthesizing metallic nanostructures with multiple morphologies and opens up another possibility for the development of customizable metallic plasmonic structures with broader applications.

Dual-functional nano-photosensitizers: Eosin-Y decorated gold nanorods for plasmon-enhanced fluorescence and singlet oxygen generation

Photosensitizer (PS) with enhanced fluorescence is attractive for image-guided photodynamic therapy (PDT) due to its dual functional role in Singlet Oxygen Generation (SOG) and producing high fluorescence signals. Here, Eosin-Y (Ey) decorated polymer coated gold nanorods (GNRs) of different aspect ratios are synthesized and introduced as novel plasmon-enhanced nano-photosensitizers for this purpose. We show, upon excitation at 519 nm, simultaneous enhancement in fluorescence and SOG was achieved for the hybrid nanostructure. The best enhancement factors of 110 and 18 for metal-enhanced fluorescence and metal-enhanced SOG, respectively, are obtained with GNRs of length 133 nm and width 45 nm, where Ey is positioned at 12.6 nm from the metal core using layer-by-layer assembly of oppositely charged polymers. The observed plasmonic effect is critically analysed by comparing the near field damping rate along with decay length, far field scattering and nonradiative energy transfer of the nanohybrids.

Coffee Ring Effect Enhanced Surface Plasmon Resonance Imaging Biosensor via 2-λ Fitting Detection Method

SPR biosensors have been extensively used for investigating protein-protein interactions. However, in conventional surface plasmon resonance (SPR) biosensors, detection is limited by the Brownian-motion-governed diffusion process of sample molecules in the sensor chip, which makes it challenging to detect biomolecule interactions at ultra-low concentrations. Here, we propose a highly sensitive SPR imaging biosensor which exploits the coffee ring effect (CRE) for in situ enrichment of molecules on the sensing surface. In addition, we designed a wavelength modulation system utilizing two LEDs to reduce the system cost and enhance the detection speed. Furthermore, a detection limit of 213 fM is achieved, which amounts to an approximately 365 times improvement compared to traditional SPR biosensors. With further development, we believe that this SPR imaging system with high sensitivity, less sample consumption, and faster detection speed can be readily applied to ultra-low-concentration molecular detection and interaction analysis.

Plasmon-Enhanced Digital Fluoroimmunoassay for Subfemtomolar Detection of Protein Biomarkers

Rapid and accurate quantification of low-abundance protein biomarkers in biofluids can transform the diagnosis of a range of pathologies, including infectious diseases. Here, we harness ultrabright plasmonic fluors as "digital nanolabels" and demonstrate the detection and quantification of subfemtomolar concentrations of human IL-6 and SARS-CoV-2 alpha and variant proteins in clinical nasopharyngeal swab and saliva samples from COVID-19 patients. The resulting digital plasmonic fluor-linked immunosorbent assay (digital p-FLISA) enables detection of SARS-CoV-2 nucleocapsid protein, both in solution and in live virions. Digital p-FLISA outperforms the "gold standard" enzyme-linked immunosorbent assay (ELISA), having a nearly 7000-fold lower limit-of-detection, and outperforms a commercial antigen test, having over 5000-fold improvement in analytical sensitivity. Detection and quantification of very low concentrations of target proteins holds potential for early detection of pathological conditions, treatment monitoring, and personalized medicine.

Curcumin-capped gold nanorods as optical sensing platform for sequence specific detection of DNA based on their self-assembly

We are reporting curcumin-induced gold nanorods as an optical sensing platform for the detection of sequence-specific DNA target through their self-assembly. The combined effect of eco-friendly reducing agent (i.e., curcumin) and silver nitrate in a basic medium (i.e., pH 10) has been attributed for the formation of small gold nanorods (AuNRs) having approximate length and diameter i.e., 19.7 ± 0.8 nm and 6.0 ± 0.5 nm, respectively, and lower longitudinal surface plasmon resonance (SPR) enable to detect and analyse different biomarkers. Further, for evaluating cellular uptake of as-synthesized AuNRs, the cytotoxicity study has been carried out by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl-2H-tetrazolium bromide (MTT) assay on A549 cells and HEPG2 cell lines, respectively, and shown approximately similar cytotoxicity. Interestingly, as-synthesized optically and electronically active AuNRs based nanobiosensing platform enable to detect sequence-specific DNA targets with low level of detection limit i.e., LOD 8.6 ± 0.15 pM for complimentary target (CT) DNA with higher sensitivity and better selectivity. Finally, this study is suggesting a simplistic bio-mediated approach of tuning the shape and size of AuNRs for sensitive, selective and reliable nanobiosensing platform for sequence-specific DNA detection related to cancer cells.

Ultrasensitive lateral-flow assays via plasmonically active antibody-conjugated fluorescent nanoparticles

Lateral-flow assays (LFAs) are rapid and inexpensive, yet they are nearly 1,000-fold less sensitive than laboratory-based tests. Here we show that plasmonically active antibody-conjugated fluorescent gold nanorods can make conventional LFAs ultrasensitive. With sample-to-answer times within 20 min, plasmonically enhanced LFAs read out via a standard benchtop fluorescence scanner attained about 30-fold improvements in dynamic range and in detection limits over 4-h-long gold-standard enzyme-linked immunosorbent assays, and achieved 95% clinical sensitivity and 100% specificity for antibodies in plasma and for antigens in nasopharyngeal swabs from individuals with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). Comparable improvements in the assay's performance can also be achieved via an inexpensive portable scanner, as we show for the detection of interleukin-6 in human serum samples and of the nucleocapsid protein of SARS-CoV-2 in nasopharyngeal samples. Plasmonically enhanced LFAs outperform standard laboratory tests in sensitivity, speed, dynamic range, ease of use and cost, and may provide advantages in point-of-care diagnostics.

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.

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.

Label-Free Multiplexed Microfluidic Analysis of Protein Interactions Based on Photonic Crystal Surface Mode Imaging

High-throughput protein assays are crucial for modern diagnostics, drug discovery, proteomics, and other fields of biology and medicine. It allows simultaneous detection of hundreds of analytes and miniaturization of both fabrication and analytical procedures. Photonic crystal surface mode (PC SM) imaging is an effective alternative to surface plasmon resonance (SPR) imaging used in conventional gold-coated, label-free biosensors. PC SM imaging is advantageous as a quick, label-free, and reproducible technique for multiplexed analysis of biomolecular interactions. PC SM sensors are characterized by a longer signal propagation at the cost of a lower spatial resolution, which makes them more sensitive than classical SPR imaging sensors. We describe an approach for designing label-free protein biosensing assays employing PC SM imaging in the microfluidic mode. Label-free, real-time detection of PC SM imaging biosensors using two-dimensional imaging of binding events has been designed to study arrays of model proteins (antibodies, immunoglobulin G-binding proteins, serum proteins, and DNA repair proteins) at 96 points prepared by automated spotting. The data prove feasibility of simultaneous PC SM imaging of multiple protein interactions. The results pave the way to further develop PC SM imaging as an advanced label-free microfluidic assay for the multiplexed detection of protein interactions.

Portable Plasmonic Paper-Based Biosensor for Simple and Rapid Indirect Detection of CEACAM5 Biomarker via Metal-Enhanced Fluorescence

Rapid, simple, and sensitive analysis of relevant proteins is crucial in many research areas, such as clinical diagnosis and biomarker detection. In particular, clinical data on cancer biomarkers show great promise in forming reliable predictions for early cancer diagnostics, although the current analytical systems are difficult to implement in regions of limited recourses. Paper-based biosensors, in particular, have recently received great interest because they meet the criteria for point-of-care (PoC) devices; the main drawbacks with these devices are the low sensitivity and efficiency in performing quantitative measurements. In this work, we design a low-cost paper-based nanosensor through plasmonic calligraphy by directly drawing individual plasmonic lines on filter paper using a ballpoint pen filled with gold nanorods (AuNR) as the colloidal ink. The plasmonic arrays were further successively coated with negatively and positively charged polyelectrolyte layers employed as dielectric spacers to promote the enhancement of the emission of carboxyl-functionalized quantum dots (QD)—previously conjugated with specific antibodies—for indirect detection of the carcinoembryonic antigen-related cell adhesion molecule 5 (CEACAM5). The efficiency, sensitivity, as well as the specificity of our portable nanosensor were validated by recording the luminescence of the QD@Ab complex when different concentrations of CEACAM5 were added dropwise onto the calligraphed plasmonic arrays.

Optimizing Plasmonic Gold Nanorod Deposition on Glass Surfaces for High-Sensitivity Refractometric Biosensing

Owing to high surface sensitivity, gold nanorods (AuNRs) are widely used to construct surface-based nanoplasmonic biosensing platforms for label-free molecular diagnostic applications. A key fabrication step involves controlling AuNR deposition onto the target surface, which requires maximizing surface density while minimizing inter-particle aggregation, and is often achieved by surface functionalization with a self-assembled monolayer (SAM) prior to AuNR deposition. To date, existing studies have typically used a fixed concentration of SAM-forming organic molecules (0.2-10% v/v) while understanding how SAM density affects AuNR deposition and resulting sensing performance would be advantageous. Herein, we systematically investigated how controlling the (3-aminopropyl)triethoxysilane (APTES) concentration (1-30% v/v) during SAM preparation affects the fabrication of AuNR-coated glass surfaces for nanoplasmonic biosensing applications. Using scanning electron microscopy (SEM) and UV-visible spectroscopy, we identified an intermediate APTES concentration range that yielded the highest density of individually deposited AuNRs with minimal aggregation and also the highest peak wavelength in aqueous solution. Bulk refractive index sensitivity measurements indicated that the AuNR configuration had a strong effect on the sensing performance, and the corresponding wavelength-shift responses ranged from 125 to 290 nm per refractive index unit (RIU) depending on the APTES concentration used. Biosensing experiments involving protein detection and antigen-antibody interactions further demonstrated the high surface sensitivity of the optimized AuNR platform, especially in the low protein concentration range where the measurement shift was ~8-fold higher than that obtained with previously used sensing platforms.

Thermally stable gold nanorod dispersed silicone composite with plasmonic resonance in the optical communication window

Gold nanorods (AuNRs) possess a high optical nonlinear coefficient, ultrafast optical response speed and widely tunable localized surface plasmon resonance (LSPR) wavelength covering the visible and near infrared region. Therefore, they are extensively investigated for many optical applications. However, the poor thermal stability of the AuNRs seriously restricts their practical performance. In addition, for many applications, such as optical communication or laser modulation, AuNRs have to be combined with transparent solids, for example polymers, glass or crystals to make devices. Here, we report on the preparation of 0.23 mg AuNR dispersed methyl silicone resin (MSR) with longitudinal LSPR (L-LSPR) wavelength (1450 nm) in the optical communication window. We found that AuNR-silicone composites possess high thermal stability. After calcination in ambient environment at a temperature of 250 °C for 10 h, the L-LSPR peak of the sample can remain longer than 1380 nm, implying that the NR shape of the Au particles was well maintained. Using the open-aperture Z-scan technique, the nonlinear absorption coefficient of the composites was measured as -11.71 cm GW^-1, higher than many nonlinear materials. Thus, the thermally stable AuNR@SiO₂-MSR composite with high nonlinearity is promising for practical applications in the optical communication window.

A Nanoarchitectonic Approach Enables Triple Modal Synergistic Therapies To Enhance Antitumor Effects

Improvement of antitumor effects relies on the development of biocompatible nanomaterials and combination of various therapies to produce synergistic effects and avoid resistance. In this work, we developed GBD-Fe, a nanoformulation that effectively integrated chemotherapy (CT), chemodynamic therapy (CDT), and photothermal therapy (PTT). GBD-Fe used gold nanorods as photothermal agents and encapsulated doxorubicin to amplify Fe³⁺-guided CDT effects by producing H₂O₂ and reducing the intracellular glutathione levels. In vitro and in vivo experiments were conducted to demonstrate the enhanced accumulation and antitumor effects of this tripronged therapy under magnetic resonance imaging (MRI) guidance. This tripronged approach of CT/CDT/PTT effectively induced tumor cytotoxicity and inhibited tumor growth in tumor-bearing mice and therefore represents a promising strategy to effectively treat tumors.

Plasmonic nanosensors for point-of-care biomarker detection

Advancement of materials along with their fascinating properties play increasingly important role in facilitating the rapid progress in medicine. An excellent example is the recent development of biosensors based on nanomaterials that induce surface plasmon effect for screening biomarkers of various diseases ranging from cancer to Covid-19. The recent global pandemic re-confirmed the trend of real-time diagnosis in public health to be in point-of-care (POC) settings that can screen interested biomarkers at home, or literally anywhere else, at any time. Plasmonic biosensors, thanks to its versatile designs and extraordinary sensitivities, can be scaled into small and portable devices for POC diagnostic tools. In the meantime, efforts are being made to speed up, simplify and lower the cost of the signal readout process including converting the conventional heavy laboratory instruments into lightweight handheld devices. This article reviews the recent progress on the design of plasmonic nanomaterial-based biosensors for biomarker detection with a perspective of POC applications. After briefly introducing the plasmonic detection working mechanisms and devices, the selected highlights in the field focusing on the technology's design including nanomaterials development, structure assembly, and target applications are presented and analyzed. In parallel, discussions on the sensor's current or potential applicability in POC diagnosis are provided. Finally, challenges and opportunities in plasmonic biosensor for biomarker detection, such as the current Covid-19 pandemic and its testing using plasmonic biosensor and incorporation of machine learning algorithms are discussed.