High-Throughput Single-Particle Analysis of Metal-Enhanced Fluorescence in Free Solution Using Ag@SiO2 Core-Shell Nanoparticles

Development of highly sensitive dual-enhanced fluorescence quenching immunochromatographic test strips based on Pt nanoprobes

The fluorescence-quenching method is crucial in vitro analysis, particularly for immunochromatographic test strips (ICTs) using noble metal nanoparticles as probes. However, ICTs still fall short in meeting the requirements for the detection of traces biomarkers due to the noble metal nanoparticles can only quench fluorescence of the dyes within a confined distance. Interestingly, noble metal nanoparticles, such as Pt NPs cannot only perform fluorescence-quenching ability based on the Förster resonance energy transfer (FRET), but also show perfect oxidase-like catalytic performance on many kinds of substrates, such as 3,3',5,5' -tetramethylbenzidine (TMB). We observed that the oxTMB (the oxidation products of TMB) exhibited notable effectiveness in quenching Cy5 fluorescence by the strong inner filter effect (IFE), which obviously improved the fluorescence-quenching efficiency with extremely low background signal. Through the dual-enhanced fluorescence quenching mechanism, the fluorescence quenching constant (Kn) was 661.24-fold that of only Pt NPs on the NC membrane. To validate the feasibility of this technique, we employed two types of biomarkers, namely microRNA (miR-15a-5p) and the signature protein (PSA). The sensitivity of miR-15a-5p was 9.286 × 10-18 mol/L and 17.5-fold more than that based on Pt NPs. As for the PSA, the LOD (0.6265 pg/mL) was 15.5-fold enhancement more sensitive after catalysis. Overall, the dual-enhanced fluorescence quenching rFICTs could act as a practical detection for biomarker in real samples.

Plasmonic-Enhanced Bright Single Spin Defects in Silicon Carbide Membranes

Optically addressable spin defects in silicon carbide (SiC) have emerged as attractable platforms for various quantum technologies. However, the low photon count rate significantly limits their applications. We strongly enhanced the brightness by 7 times and spin-control strength by 14 times of single divacancy defects in 4H-SiC membranes using a surface plasmon generated by gold film coplanar waveguides. The mechanism of the plasmonic-enhanced effect is further studied by tuning the distance between single defects and the surface of the gold film. A three-energy-level model is used to determine the corresponding transition rates consistent with the enhanced brightness of single defects. Lifetime measurements also verified the coupling between defects and surface plasmons. Our scheme is low-cost, without complicated microfabrication and delicate structures, which is applicable for other spin defects in different materials. This work would promote developing spin-defect-based quantum applications in mature SiC materials.

Competitive fluorescent immunoassay for the ultrasensitive determination of amyloid beta peptide1-42 based on Ag@SiO2@N, S-GQD nanocomposites

A competitive fluorescent immunoassay is described for the ultrasensitive determination of amyloid beta peptide1-42 (Aβ1-42), a biomarker for early diagnosis of Alzheimer's disease. N, S-doped graphene quantum dots (N, S-GQDs) were freely assembled on the surface of Ag@SiO₂ nanoparticles to obtain a composite (Ag@SiO₂@N, S-GQD nanocomposite), which was successfully prepared and characterized. By theoretical study, the optical properties of nanocomposites are improved compared with GQDs, due to the advantages of combining N, S co-doping and metal-enhanced fluorescence (MEF) effect of Ag NPs. In addition, Aβ1-42 was modified by Ag@SiO₂@N, S-GQDs to prepare a probe with high photoluminescence properties (Ag@SiO₂@N, S-GQDs-Aβ1-42). In the presence of Aβ1-42, a competitive reaction towards anti-Aβ1-42 fixed on the ELISA plate was proceeded between Aβ1-42 and Ag@SiO₂@N, S-GQDs-Aβ1-42 by specific capture of antigen-antibody. The emission peak of Ag@SiO₂@N, S-GQDs-Aβ1-42 (400 nm emission) was used for the quantitative determination of Aβ1-42. Under the optimal conditions, the fluorescent immunoassay exhibited a linear range of 0.32 pg·mL^-1-5 ng·mL^-1 with a detection limit of 0.098 pg·mL^-1. The results show that the immunoassay has good analytical ability and can provide a new method for the clinical determination of Aβ1-42.

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.

A fluorescent sensor based on the cascade signal amplification strategy for ultra-sensitive detection of Cu2

Copper is an essential element in the human body, participating in various physiological activities in the bodies of organisms. However, an excessive load of Cu²⁺ is associated with several neurodegenerative diseases and prion diseases, also identified as a symptom of Wilson's disease (WD). A straightforward, rapid, sensitive, and specific copper sensor is highly required but remains a challenge. In this study, guided by the simulation, we developed a chemical sensor using a cascade signal amplification strategy based on the Cu-catalyzed click reaction, combined with a fluorescence-enhanced substrate with gold nanorods coupled with silver nanoislands. The sensor can selectively detect Cu²⁺ as low as 3.87 nM within 10 min. We have demonstrated that this method can be directly employed for WD diagnosis in urine samples. In addition, using antibiotic susceptibility testing (AST) as an example, we verify whether this assay can be adapted to other targets where Cu is designed as an indirect indicator.

Single-Particle Measurements of Nanocatalysis with Dark-Field Microscopy

Due to the complexity of heterogeneous reactions and heterogeneities of individual catalyst particles in size, morphology, and the surrounding medium, it is very important to characterize the structure of nanocatalysts and measure the reaction process of nanocatalysis at the single-particle level. Traditional ensemble measurements, however, only provide averaged results of billions of nanoparticles (NPs), which do not help reveal structure–activity relationships and may overlook a few NPs with high activity. The advent of dark-field microscopy (DFM) combined with plasmonic resonance Rayleigh scattering (PRRS) spectroscopy provides a powerful means for directly recording the localized surface plasmon resonance (LSPR) spectrum of single plasmonic nanoparticles (PNPs), which also enables quantitative measurements. In recent years, DFM has developed rapidly for a series of single-particle catalytic reactions such as redox reactions, electrocatalytic reactions, and DNAzyme catalysis, with the ability to monitor the catalytic reaction process in real time and reveal the catalytic mechanism. This review provides a comprehensive overview of the fundamental principles and practical applications of DFM in measuring various kinds of catalysis (including chemocatalysis, electrocatalysis, photocatalysis, and biocatalysis) at the single-particle level. Perspectives on the remaining challenges and future trends in this field are also proposed.

Metal-Enhanced Hg2+-Responsive Fluorescent Nanoprobes: From Morphological Design to Application to Natural Waters

Metal-enhanced fluorescence (MEF) is a powerful tool in the design of sensitive chemical sensors by improving brightness and photostability of target-responsive fluorophores. Compounding these advantages with the modest hardware requirements of fluorescence sensing compared to that of centralized elemental analysis instruments, thus expanding the use of MEF to the detection of low-level inorganic pollutants, is a compelling aspiration. Among the latter, monitoring mercury in the environment, where some of its species disseminate through the food chain and, in time, to humans, has elicited a broad research effort toward the development of Hg²⁺-responsive fluorescent sensors. Herein, a Hg²⁺-sensitive MEF-enabled probe was conceived by grafting a Hg²⁺-responsive fluorescein derivative to concentric Ag@SiO₂ NPs, where the metallic core enhances fluorescence emission of molecular probes embedded in a surrounding silica shell. Time-resolved fluorescence measurements showed that the fluorophore's excited-state lifetime decreases from 3.9 ns in a solid, coreless silica sphere to 0.4 ns in the core-shell nanoprobe, granting the dye a better resistance to photobleaching. The Ag-core system showed a sizable improvement in the limit of detection at 2 nM (0.4 ppb) compared to 50 nM (10 ppb) in silica-only colloids, and its effectiveness for natural water analysis was demonstrated. Overall, the reported nanoarchitecture hints at the potential of MEF for heavy metal detection by fluorescence detection.

Nanoparticle core size and spacer coating thickness dependence on metal-enhanced luminescence in optical oxygen sensors

Metal-enhanced luminescence (MEL) originated from near field interactions of luminescence with the surface plasmon resonance (SPR) of nearby metallic nanoparticles (NPs) is an effective strategy to increase luminescence detection sensitivity in oxygen sensors. Once the excitation light induces SPR, the generated enhanced local electromagnetic filed will result in an enhanced excitation efficiency and an increased radiative decay rates of luminescence in close proximity. Meanwhile, the nonradioactive energy transfer from the dyes to the metal NPs, leading to emission quenching, can also be affected by their separation. The extent of the intensity enhancement depends critically on the particle size, shape and the separation distance between the dye and the metal surface. Here, we prepared core-shell Ag@SiO₂ with different core sizes (35 nm, 58 nm and 95 nm) and shell thickness (5-25 nm) to investigate the size and separation dependence on the emission enhancement in oxygen sensors at 0-21% oxygen concentration. Intensity enhancement factors of 4-9 were observed with a silver core size of 95 nm and silica shell thickness of 5 nm at 0-21% O₂. In addition, the intensity enhancement factor increases with increasing core size and decreasing shell thickness in the Ag@SiO₂-based oxygen sensors. Using Ag@SiO₂ NPs result in brighter emission throughout the 0-21% oxygen concentration. Our fundamental understanding of MEP in the oxygen sensors provides us the opportunity to design and control efficient luminescence enhancement in oxygen and other sensors. .

Backscattering-Based Discrimination of Microparticles Using an Optofluidic Multiangle Scattering Chip

In this research, we designed and fabricated an optofluidic chip for the detection and differentiation of single particles via the combination of backscattered (BSC) and forward-scattered (FSC) or side-scattered (SSC) light intensity. The high sensitivity of BSC light to the refractive index of the particles enabled an effective approach for the differentiation of individual particles based on the type of material. By recording BSC as well as FSC and SSC light intensities from single particles, transiting through the illumination zone in a microfluidic channel, the size and type of material could be detected simultaneously. The analysis of model samples of polystyrene (PS), as a primary microplastic particle, and silica microspheres showed substantially higher BSC signal values of PS because of a larger refractive index compared to the silica. The scatter plots correlating contributions of BSC (FSC-BSC and SSC-BSC) allowed a clear differentiation of PS and silica particles. To demonstrate the great potential of this methodology, two "real-life" samples containing different types of particles were tested as application examples. Commercial toothpaste and peeling gel products, as primary sources of microplastics into effluents, were analyzed via the optofluidic chip and compared to results from scanning electron microscopy. The scattering analysis of the complex samples enabled the detection and simultaneous differentiation of particles such as microplastics according to their differences in the refractive index via distinctive areas of high and low BSC signal values. Hence, the contribution of BSC light measurements in multiangle scattering of single particles realized in an optofluidic chip opens the way for the discrimination of single particles in a liquid medium in manifold fields of application ranging from environmental monitoring to cosmetics.

Indium nanoparticle-based surface enhanced fluorescence from deep ultraviolet to near-infrared: A theoretical study

Herein, for the first time, we report a theoretical investigation on Indium nanoparticle-based surface enhanced fluorescence (SEF) from deep ultraviolet (UV) to near-infrared (NIR). In the beginning, the far- and near-field plasmonic properties of the Indium nanospheres of different sizes are studied to extract the wavelengths of lower and higher-order localized surface plasmon modes and the corresponding local electric field enhancement (EFE) values. Later, the dependence of the SEF enhancement with the separation between the fluorophore and nanoparticle (d), fluorescence, and excitation wavelengths is studied systematically. The role of the surrounding medium on plasmon mode wavelength and the SEF enhancement is also shown. Moreover, the effect of d and fluorescence wavelength on the average SEF enhancement is investigated. Finally, the variation in the plasmonic properties after thin dielectric coating on the surface of single Indium nanospheres is studied.

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.

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.

Plasmonic Core-Shell Nanoparticle Enhanced Spectroscopies for Surface Analysis

Probing the properties and components of reactive surfaces is crucial for illustrating reaction mechanisms. However, common surface analysis techniques are restricted to in situ acquisition of surface information at the molecular scale in the human environment and industrial catalysis processes. Plasmonic spectroscopies are promising tools to solve this problem. This Feature is intended to introduce the plasmonic core-shell nanoparticle enhanced spectroscopies for qualitatively and quantitatively analyzing surface trace species. Four different working modalities are designed for meeting varied needs, involving in situ surface species detection, catalytic process monitoring, labeled sensing, and dual mode analysis. These newly developed plasmonic spectroscopies show great potential not only in fundamental research but also in practical applications.

MSNs-Based Nanocomposite for Biofilm Imaging and NIR-Activated Chem/Photothermal/Photodynamic Combination Therapy

Bacterial infections caused by biofilms are severe clinical problems, resulting in high drug resistance by limiting the penetration of antibiotics. Herein, a near-infrared (NIR)-activated chem/photodynamic/photothermal combined therapeutic agent is proposed by loading fluorescein isothiocyanate (FITC), ultrasmall copper sulfide nanoparticles (Cu_2-xSNPs), and ε-polylysine (PLL) onto mesoporous silica nanoparticles (MSNs) through a layer-by-layer self-assembly approach. FITC-doped MSNs are prepared to monitor the permeability and accumulation of nanocomposites into biofilms. MSNs can also act as hosts for the synthesis of ultrasmall Cu_2-xSNPs, which has effective photodynamic and photothermal ablation against bacteria under NIR light irradiation. Moreover, biodegradable PLL introduced can not only enhance adhesion toward the bacterial surface to increase the effectiveness of phototherapy but also damage bacteria through electrostatic interaction. As a result, the prepared nanocomposites could not only penetrate biofilms but also ablate biofilms through combined chem/photodynamic/photothermal effects under NIR light irradiation. Furthermore, the nanocomposites could treat bacterial infections in vivo with negligible tissue toxicity. Overall, the finely designed nanocomposites are anticipated to display promising applications in imaging-guided chem/photodynamic/photothermal combined therapy for bacterial infections.

Controlling plasmon propagation and enhancement via reducing agent in wet chemistry synthesized silver nanowires

Silver nanowires with varying diameters and submillimeter lengths were obtained by changing a reducing agent used during hydrothermal synthesis. The control over the nanowire diameter turns out to play a critical role in determining their plasmonic properties, including fluorescence enhancement and surface plasmon polariton propagation. Advanced fluorescence imaging of hybrid nanostructures assembled of silver nanowires and photoactive proteins indicates longer propagation lengths for nanowires featuring larger diameters. At the same time, with increasing diameter of the nanowires, we measure a substantial reduction of fluorescence enhancement. The results point at possible ways to control the influence of plasmon excitations in silver nanowires by tuning their morphology.

Macroscopic two-dimensional monolayer films of gold nanoparticles: fabrication strategies, surface engineering and functional applications

In the last few decades, two-dimensional monolayer films of gold nanoparticles (2D MFGS) have attracted increasing attention in various fields, due to their superior attributes of macroscopic size and accessible fabrication, controllable electromagnetic enhancement, distinctive optical harvesting and electron transport capabilities. This review will focus on the recent progress of 2D monolayer films of gold nanoparticles in construction approaches, surface engineering strategies and functional applications in the optical and electric fields. The research challenges and prospective directions of 2D MFGS are also discussed. This review would promote a better understanding of 2D MFGS and establish a necessary bridge among the multidisciplinary research fields.

Electrorheological Sensor Encapsulating Microsphere Media for Plague Diagnosis with Rapid Visualization

Plague is a disease infected by an etiological agent, which is transmitted from fleas to a variety of wildlife rodents. Therefore, rapid diagnosis of plague on-site in the field is important. Polystyrene microspheres (SMs) of 2.2 μm diameter were synthesized by emulsion polymerization to adsorb magnetic nanoparticles (FNs), resulting in core-/shell-structured microspheres that generate a significant contrast in relative permittivities between SMs and FNs. Electrorheological displays (EDs) consisting of two indium tin oxide glasses with spacers were constructed to contain core-/shell-structured SM/FN (SM@FN) solutions for observing their transmittance change. The ED encapsulating dispersed SM@FN solution exhibited an opaque state because light was scattered significantly without the application of an alternating electric field (AEF). In the presence of an AEF, the particle chaining behavior results in enhancement of the transmittance of ED. At a specific frequency, the so-called characteristic frequency (F_c), the transmittance reaches a maximum. F_c could be used as an indicator to mark the shell materials. The antibody of Yersinia pestis (ab-Yp) was coated onto the SM@FN as a biosensing medium. The F_c of ab-Yp-modified microspheres shifted from 200 to 750 kHz with antigen coupling of Y. pestis antigen (ag-Yp). In the absence of fluorescence labeling, the large change in ED transmittance could be visualized during the Y. pestis detection. The limit of detection and the limit of quantification were ∼30 and ∼40 ng/μL, respectively, obtained within 30 s according to the highest transmittance of ED under the AEF at 750 kHz. Y. pestis detection was not affected by Escherichia coli and Staphylococcus aureus significantly. Compared with other common immunoassays, including the secondary immunochemical or enzyme-linked steps, this simple electrorheological sensor with high sensitivity and selectivity could be a candidate for on-site plague diagnosis.

Surface curvature-dependent adsorption and aggregation of fluorescein isothiocyanate on gold nanoparticles

The interaction between metallic nanoparticles and fluorescent molecules and its influence on the optical properties of the particles/molecules have been intensively investigated because of their biology and sensing applications. Here, we studied the adsorption and aggregation of a commonly used dye, fluorescein isothiocyanate (FITC), on gold nanoparticles of various diameters. It was observed that the adsorption of FITC on relatively large gold nanoparticles (≥15 nm in diameter) induced quenching in the two-photon fluorescence (TPF) emission from the FITC molecules, while smaller-sized gold nanoparticles (1.6 nm) had no such effect. This difference was interpreted by the fluorescence resonance energy transfer (FRET) between the FITC molecules and the larger gold nanoparticles. At the same time, it was observed that the ratio of TPF quenching was notably higher than the ratio of the FITC molecules chemically adsorbed on the large gold particles. This unexpected observation revealed that the aggregation-induced fluorescence quenching also contributed significantly to the attenuation of the TPF emission. Time-dependent TPF attenuation during the interaction of FITC and the larger gold nanoparticles was recorded and used to confirm this interpretation. With this experimental evidence, a clear picture of the interaction of the FITC molecules on the gold surface was presented: FITC molecules chemically adsorbed on the small gold nanoparticles. However, the relatively larger surface curvature hindered the aggregation of the FITC molecules on the small gold nanoparticles. On the surface of the larger gold nanoparticles, both adsorption and aggregation occured. The influence of the surface curvature on the interfacial structure of the adsorbed molecules on nanoparticles was discussed.

Surface modification of plasmonic noble metal-metal oxide core-shell nanoparticles

A comprehensive survey on the methods for the surface modification of plasmonic noble metal-metal oxide core-shell nanoparticles is presented. The review highlights various strategies for covalent attachment and electrostatic binding of molecules and molecular ions to core-shell nanoparticles with a focus on plasmonically active silver and gold nanoparticles encapsulated by SiO₂ and TiO₂ shells.

Autoluminescence-Free Prostate-Specific Antigen Detection by Persistent Luminous Nanorods and Au@Ag@SiO2 Nanoparticles

The sensitive detection of biomarker with high selectivity and accuracy is of remarkable significance for the early screening and clinical diagnosis of malignant cancer. Persistent phosphors play a role in illumination-free biosensing owing to their unparalleled optical properties, which produces long-lasting luminescence without the illumination to eliminate the interference of autofluorescence and scattering light from biological matrix. In this study, ZnGeO:Mo persistent luminescence nanorods (PLNRs) were prepared and showed tunable luminescence intensity and decay patterns. An excitation-free luminescent aptasensor was proposed for the detection of prostate specific antigen (PSA) by using ZnGeO:Mo PLNRs as probes and Au@Ag@SiO₂ nanoparticles as quenchers. Under optimal conditions, the limit of detection for PSA was 9.2 pg mL^-1. This work creates a possibility for the preparation of multiplex colored persistent luminescence nanomaterials with desired decay patterns for multiple bioassay and time-resolved fluoroimmunoassay.

Optical aptasensing of mercury(II) by using salt-induced and exonuclease I-induced gold nanoparticle aggregation under dark-field microscope observation

An optical method for determination of Hg(II) is described that exploits the aggregation of gold nanoparticles (AuNPs) under dark-field microscope (DFM) observation. This assay is based on the use of a Hg(II)-specific aptamer, AuNPs modified with complementary DNA strands, and exonuclease I (Exo I). In the absence of Hg(II), the added dsDNA prevents salt-induced aggregation of the green-colored AuNPs. If Hg(II) is added, the aptamer will capture it to form T-Hg(II)-T pairs, and the complementary strand is digested by Exo I. On addition of a solution of NaCl, the AuNPs will aggregate. This is accompanied by a color change from green to orange/red) in the dark-field image. By calculating the intensity of the orange/red dots in the dark-field image, concentration of Hg(II) can be accurately determined. The limit of detection is as low as 36 fM, and response is a linear in the 83 fM to 8.3 μM Hg(II) concentration range. Graphical abstract Schematic representation of a colorimetric assay for Hg(II) based on the use of a mercury(II)-specific aptamer, gold nanoparticles modified with complementary DNA strands, and exonuclease I.

Hybrid Silver Nanocubes for Improved Plasmon-Enhanced Singlet Oxygen Production and Inactivation of Bacteria

Plasmonic nanoparticles can strongly interact with adjacent photosensitizer molecules, resulting in a significant alteration of their singlet oxygen (¹O₂) production. In this work, we report the next generation of metal-enhanced ¹O₂ nanoplatforms exploiting the lightning rod effect, or plasmon hot spots, in anisotropic (nonspherical) metal nanoparticles. We describe the synthesis of Rose Bengal-decorated silica-coated silver nanocubes (Ag@SiO₂-RB NCs) with silica shell thicknesses ranging from 5 to 50 nm based on an optimized protocol yielding highly homogeneous Ag NCs. Steady-state and time-resolved ¹O₂ measurements demonstrate not only the silica shell thickness dependence on the metal-enhanced ¹O₂ production phenomenon but also the superiority of this next generation of nanoplatforms. A maximum enhancement of ¹O₂ of approximately 12-fold is observed with a 10 nm silica shell, which is among the largest ¹O₂ production metal enhancement factors ever reported for a colloidal suspension of nanoparticles. Finally, the Ag@SiO₂-RB NCs were benchmarked against the Ag@SiO₂-RB nanospheres previously reported by our group, and the superior ¹O₂ production of Ag@SiO₂-RB NCs resulted in improved antimicrobial activities in photodynamic inactivation experiments using both Gram-positive and -negative bacteria model strains.

Color-Coded Single-Particle Pyrophosphate Assay with Dark-Field Optical Microscopy

In this work, we demonstrate a convenient yet sensitive color-coded single-particle detection method for the quantification of pyrophosphate (PPi) by using single gold nanoparticle (GNP) as the probe. The design is based on GNP-dependent catalytic deposition of Cu onto the surface of GNPs with reduced nicotinamide adenine dinucleotide (NADH). Without PPi, Cu²⁺ can be directly reduced to Cu⁰ through the gold-catalyzed oxidization of NADH. In the presence of PPi, the coating process is impeded due to the strong coordination capability of PPi with Cu²⁺. The selective coating of Cu shell onto the GNPs surface results in the extraordinary red-shift of localized surface plasmon resonance from individual GNPs. By quantitatively counting the fraction of yellow particles with color-coded dark-field optical microscopy, the trace amounts of PPi in solution can be accurately quantified. The limit-of-detection is as low as 1.49 nM with a linear dynamic range of 0-4.29 μM, which is much lower than the spectroscopic measurements in bulk solution. In artificial urine sample, good recovery efficiency was achieved. As a consequence, the method demonstrated herein will find promising applications for the ultrasensitive detection of target biomolecules under biological milieu in the future.

Plant Biomarker Recognition by Molecular Imprinting Based Localized Surface Plasmon Resonance Sensor Array: Performance Improvement by Enhanced Hotspot of Au Nanostructure

Detection of plant volatile organic compounds (VOCs) enables monitoring of pests and diseases in agriculture. We previously revealed that a localized surface plasmon resonance (LSPR) sensor coated with a molecularly imprinted sol-gel (MISG) can be used for cis-jasmone vapor detection. Although the selectivity of the LSPR sensor was enhanced by the MISG coating, its sensitivity was decreased. Here, gold nanoparticles (AuNPs) were doped in the MISG to enhance the sensitivity of the LSPR sensor through hot spot generation. The size and amount of AuNPs added to the MISG were investigated and optimized. The sensor coated with the MISG containing 20 μL of 30 nm AuNPs exhibited higher sensitivity than that of the sensors coated with other films. Furthermore, an optical multichannel sensor platform containing different channels that were bare and coated with four types of MISGs was developed to detect plant VOCs in single and binary mixtures. Linear discriminant analysis, k-nearest neighbor (KNN), and naïve Bayes classifier approaches were used to establish plant VOC identification models. The results indicated that the KNN model had good potential to identify plant VOCs quickly and efficiently (96.03%). This study demonstrated that an LSPR sensor array coated with a AuNP-embedded MISG combined with a pattern recognition approach can be used for plant VOC detection and identification. This research is expected to provide useful technologies for agricultural 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.