Nitrite-Triggered Surface Plasmon-Assisted Catalytic Conversion of p-Aminothiophenol to p,p′-Dimercaptoazobenzene on Gold Nanoparticle: Surface-Enhanced Raman Scattering Investigation and Potential for Nitrite Detection

Surface-Enhanced Raman Spectroscopy for Nitrite Detection

Nitrite is an important chemical intermediate in the nitrogen cycle and is ubiquitously present in environmental and biological systems as a metabolite or additive in the agricultural and food industries. However, nitrite can also be toxic in excessive concentrations. As such, the development of quick, sensitive, and portable assays for its measurement is desirable. In this review, we summarize the working principles and applications of surface-enhanced Raman spectroscopy (SERS) as a rapid, portable, and ultrasensitive method for nitrite detection and showcase its applicability in various water, food, and biological samples. The challenges and opportunities for future developments are also discussed.

Internal standard optimization advances sensitivity and robustness of ratiometric detection method

We design a p-aminothiophenol (pATP) modified Au/ITO chip to determine nitrite ions in lake water by a ratiometric surface-enhanced Raman scattering (SERS) method based on nitrite ions triggering the transformation of pATP to p,p'-dimercaptoazobenzene (DMAB). Intriguingly, by using the SERS peak (at 1008 cm-1) from benzoic ring deforming as an internal standard instead of the traditional peak at 1080 cm-1, the detection sensitivity of the method was improved 10 times.

Surface plasmon-assisted catalytic reduction of p-nitrothiophenol for the detection of Fe2+ by surface-enhanced Raman spectroscopy

Herein, we developed a concise, time-efficient, and high selective assay for detecting Fe2+ through its triggered surface plasmon-assisted reduction reaction of p-nitrothiophenol (PNTP) to p,p'-dimercaptoazobenzene (DMAB) on the surface of gold nanoparticles (AuNPs) based on surface-enhanced Raman scattering (SERS) spectroscopy. When Fe2+ was added to the PNTP-AuNPs system, the appearance of three characteristic peaks at 1142, 1392, and 1440 cm-1 attributed to DMAB demonstrated that Fe2+ induced the catalytic coupling reaction of PNTP. The Raman intensity ratio of the peak at 1142 cm-1 to the peak at 1336 cm-1 and the concentration of Fe2+ presented a good linear response from 10 to 100 μM with a limit of detection (LOD) of 0.35 μM. More importantly, the entire detection process can be completed within 2 min and further successfully used for the detection of Fe2+ in river water.

Novel Ag-coated nanofibers prepared by electrospraying as a SERS platform for ultrasensitive and selective detection of nitrite in food

Nitrite is commonly used as a preservative and color fixative in the meat industry. However, the risk of it transforming into N-nitrosamine restricts its intake. Herein, a novel sensitive Ag-coated nanofiber surface-enhanced Raman scattering (SERS) platform was developed for rapid nitrite detection. The electrospraying technique was firstly used to assemble Ag nanoparticles (NPs) on the nanofibers to obtaine SERS platform. The homogeneity and long-term stability of the SERS platform were evaluated. The limit of detection (LOD) of the SERS platform was estimated to be 2.216 × 10^-12 mol/L, corresponding to 15.29 ng·L^-1 and good linearity was shown between the relative SERS intensity and nitrite concentration range of 10-1 to 10^-4 mol/L. The Ag-coated nanofiber SERS platform was utilized to assay-five common nitrite foods, and the results provided valid evidence for the compatibility of SERS platform in quantitative nitrite detection.

Sulfite-triggered surface plasmon-catalyzed reduction of p-nitrothiophenol to p,p'-dimercaptoazobenzene

The conversion of p-aminothiophenol (PATP) or p-nitrothiophenol (PNTP) to p,p'-dimercaptoazobenzene (DMAB) has been used as model reactions to study plasmon-catalyzed reaction on nanoparticles. Herein, we report the conversion of PNTP to DMAB which is triggered by SO₃^2- ions on gold nanoparticles (AuNPs) for the first time. With the addition of SO₃^2-, the Raman peaks at 1139, 1392, 1437 cm^-1 appears, which indicates the formation of DMAB. The experiment results suggested that the synergistic effect of AuNPs and SO₃^2- promoted the conversion of PNTP to DMAB. Besides, the proposed catalysis system is high selectivity to SO₃^2- ions, which provides a new detection route to SO₃^2- ions in the future. More importantly, the possible reaction mechanism has been put forward which is helpful to understand the surface plasmon-assisted catalytic reduction of PNTP on the surface of SERS substrate.

Instant Preparation of Ultraclean Gold Nanothorns under Ambient Conditions for SERS Kit-Enabled Mobile Diagnosis

Availability of surface-enhanced Raman scattering (SERS) substrates with good stability, high sensitivity, and a clean surface is crucial for the practical usefulness of the SERS technology in biochemical sensing, especially for point-of-care testing (POCT). Hereby, we develop a "ready-to-use" SERS kit, which requires only 20 s to fabricate ultraclean gold nanothorn (AuNT)-based SERS chips under ambient conditions with simple solution processing steps. By varying the thickness of the pre-coated platinum (Pt) nanolayer, we can control the size and number density of the grown AuNT. Taking advantage of the ultraclean surface of the instantly obtained fresh AuNT, Raman reporter molecules can also be immediately modified, by means of which specific detection of three analytes including H₂O₂, NO₂^-, and ClO^- is realized. Furthermore, we propose the concept of an SERS kit and apply it to smartphone-based Raman analysis for POCT applications. This on-site preparation method solves the long-standing challenges hindering the practical use of SERS substrates, such as complicated fabrication processes, interference of residual surfactants, poor surface stability, and easy contamination. Besides performing SERS analysis conveniently and quickly, this SERS kit-enabled POCT technology can integrate remote data terminals and medical resources, which shows great potential for environmental protection or online-healthcare systems.

Photocatalytic Properties of Silver Nanospherical Arrays Driven by Surface Plasmons

Surface-enhanced Raman scattering (SERS) is a promising technique to study the plasma-driven photocatalytic reactions. Hemispherical alumina nanoarrays with a regular hexagonal arrangement are firstly prepared; then, silver hemispherical nanoarrays are synthesized on the surface of the arrays by silver evaporation. When a laser with a specific wavelength (633 nm) is irradiated on the silver nanoarrays, a large number of regularly arranged local surface plasmon enhancement regions (called “hot spots”) would be generated on its surface. After that, a layer of evenly distributed p-aminothiophenol (PATP) probe molecules was placed on the substrate and the photocatalytic reaction of PATP was driven by the local surface plasmon to form four 4′-di-mercaptoazobenzene (DMAB). Then, under the same experimental conditions, the later product was reversely reacted to form PATP molecule by the action of plasma in the presence of in situ sodium borohydride. SERS can be used to monitor the whole process of the photocatalytic reaction of PATP probe molecules driven by the plasma on the surface of the silver nanoarrays. This research achieves the drawing and erasing of molecular graphics in the micro- and nano-scales, as well as information encryption, reading, and erasing that have strong application value.

Point-of-Care Detection of Salivary Nitrite Based on the Surface Plasmon-Assisted Catalytic Coupling Reaction of Aromatic Amines

Rapid quantification of nitrite (NO₂^-) in food, drink and body fluids is of significant importance for both food safety and point-of-care (POA) applications. However, conventional nitrite analytical methods are complicated, constrained to sample content, and time-consuming. Inspired by a nitrite-triggered surface plasmon-assisted catalysis (SPAC) reaction, a rapid point-of-care detection salivary nitrate was developed in this work. NO₂^- ions can trigger the rapid conversion of p-aminothiophenol (PATP) to p,p'-dimercaptozaobenzene (DMAB) on gold nanoparticles (GNPs) under light illumination, and the emerged new bands at ca. 1140, 1390, 1432 cm^-1 originating from DMAB can be used to the quantification of nitrite. Meanwhile, to make the method entirely suitable for on-site fast screen or point-of-care application, the technique is needed to be further optimized. The calibration graph for nitrates was linear in the range of 1-100 µM with a correlation coefficient of 0.9579. The limit of detection was 1 µM. The facile method could lead to a further understanding of the progression and treatment of periodontitis and to guide professionals in planning on-site campaigns to effectively control periodontal diseases.

Ligand-Supported Hot Electron Harvesting: Revisiting the pH-Responsive Surface-Enhanced Raman Scattering Spectrum of p-Aminothiophenol

The discussion of the surface-enhanced Raman scattering (SERS) spectra of p-aminothiophenol (PATP) and of its photocatalytic reaction product 4,4'-dimercaptoazobenzene (DMAB) is important for understanding plasmon-supported spectroscopy and catalysis. Here, SERS spectra indicate that DMAB forms also in a nonphotocatalytic reaction on silver nanoparticles. Spectra measured at low pH, in the presence of the acids HCl, H₂SO₄, HNO₃, and H₃PO₄, show that DMAB is reduced to PATP when both protons and chloride ions are present. Moreover, the successful reduction of DMAB in the presence of other, halide and nonhalide, ligands suggests a central role of these species in the reduction. As discussed, the ligands increase the efficiency of hot-electron harvesting. The pH-associated reversibility of the SERS spectrum of PATP is established as an observation of the DMAB dimer at high pH and of PATP as a product of its hot-electron reduction at low pH, in the presence of the appropriate ligand.

In situ monitoring of catalytic reaction on single nanoporous gold nanowire with tuneable SERS and catalytic activity

Single nanoporous gold nanowire was introduced as a tunable one-dimensional nano-sensor platform with both SERS and catalytic activity, and it precisely fit the requirement of materials for in situ SERS monitoring of plasmon-assisted catalytic reaction. The nanoporous gold nanowires exhibited much more "hot spots" on their surface and much better SPR effect than the smooth nanowires. We demonstrated that these nanowires could be used as a SERS substrate assuring the sensitivity and reproducibility of Raman signals. Besides, they could be applied as a kind of heterogeneous catalyst for in situ SERS monitoring of the plasmon-assisted catalytic reaction-reduction of p-nitrothiophenol (p-NTP) to p,p-dimercaptoazobenzene (DMAB) at their surface. The SERS and catalytic activity of the nanowires could be respectively optimized by adjusting their dealloying time, similar to the procedure of catalyst screening.

Surface plasmon-catalyzed oxidation of 4-aminodiphenyl disulfide for determination of Ag+ ion in aqueous samples

A new sensor for determination of Ag⁺ ion (Ag⁺) by surface-enhanced Raman scattering (SERS) is reported. Gold nanoparticles (AuNPs) and 4-aminodiphenyl disulfide (APDS) were chosen as the SERS substrate and probe molecule, respectively. With the addition of Ag⁺, three new peaks (1141, 1392, and 1435 cm^-1) appeared in the SERS spectrum, indicating that the conversion of APDS to p,p'-dimercaptoazobenzene (DMAB) was achieved. As the concentration of Ag⁺ increased, the conversion of APDS to DMAB also increased and showed a good linear relationship (R² = 0.9746) in the range of 10 to 100 μM of Ag⁺. The limit of detection (LOD) was 7 μM. Compared with the traditional determination method, the SERS method is convenient and fast and requires no complicated preprocessing. Graphical abstract.

Using Diazotization Reaction to Develop Portable Liquid-Crystal-Based Sensors for Nitrite Detection

A liquid-crystal (LC)-based sensor for detecting nitrite in aqueous solutions was developed using a diazotization reaction as the sensing mechanism. First, tetradecyl 4-aminobenzoate (14CBA) was synthesized and doped into a nematic LC, i.e., 4-cyano-4'-pentylbiphenyl (5CB). When the LC mixture was cast on a glass substrate and then immersed into an aqueous solution without nitrite, the orientation of LC was planar and the LC image was bright. In the presence of nitrite, it reacted with alkylanilines to give corresponding diazonium ions with a positive charge, which aligned at the LC/aqueous interface to cause homeotropic orientation of LC. As a result, a bright-to-dark transition of the LC image was observed. The limit of detection (LOD) of this system for nitrite is 25 μM with high selectivity. In addition, this system can work in environmental water samples such as tap water and pond water. Finally, we demonstrated that the optical signals of LC can be measured and recorded using a built-in digital camera of a smartphone, suggesting the portability of this system for on-site applications.

Fluorescent and colorimetric dual-readout sensor based on Griess assay for nitrite detection

In this work, we presented a sensitive and selective colorimetric and fluorescent dual-readout sensor based on Griess assay for nitrite (NO₂^-) detection under acidic condition. The sensor system was constituted of acid-resistant carbon quantum dots (CQDs) and 3-aminophenol (3-Aph) with acidic condition regulated using HCl. During the sensing procedure, reaction of 3-Aph and NO₂^- under acidic condition can yield a yellow-colored azoic compound (AZO), which gives the colorimetric readout; meanwhile, the fluorescence of CQDs (fluorescence reporter) quenched due to the strong absorption of AZO, leading to fluorescent readout. Wherein, CQDs were synthesized via hydrothermal method through using polyacrylamide as precursor and characterized by AFM, XRD and XPS. Under the optimized condition, the sensor exhibit broad linear relationships towards NO₂^- in the range of 10 to 100 nM and 2.5 to 100 μM, with practical detection limits of 10 nM and 2.5 μM for the fluorescent and colorimetric readout, respectively. And the sensor displayed excellent capability of selectivity according to interferences study. Furthermore, testing of sprouts, bacon and ham sausage real samples demonstrated good recoveries and reproducibility of the sensor system. All these results suggested the presented colorimetric and fluorescent dual-readout sensor can be a promising candidate for the NO₂^- detection in real applications.

Review of Experimental Setups for Plasmonic Photocatalytic Reactions

Plasmonic photocatalytic reactions have been substantially developed. However, the mechanism underlying the enhancement of such reactions is confusing in relevant studies. The plasmonic enhancements of photocatalytic reactions are hard to identify by processing chemically or physically. This review discusses the noteworthy experimental setups or designs for reactors that process various energy transformation paths for enhancing plasmonic photocatalytic reactions. Specially designed experimental setups can help characterize near-field optical responses in inducing plasmons and transformation of light energy. Electrochemical measurements, dark-field imaging, spectral measurements, and matched coupling of wavevectors lead to further understanding of the mechanism underlying plasmonic enhancement. The discussions herein can provide valuable ideas for advanced future studies.

Isomerization of p,p'-Diiodoazobenzene Controlled by the Surface Plasmon-Assisted Reaction

Based on a SERS study, the plasmon-assisted coupling reaction of p-iodoaniline (PIAN) to p,p'-diiodoazobenzene was investigated. This model plasmon-assisted reaction exhibits an outstanding SERS performance with the limit of detection as low as 10-13 M. Under low concentrations, the control of the cis-trans isomerization of p,p'-diiodoazobenzene was achieved by characterizing the Raman peak intensity at 1432 cm-1 of the coupling reaction catalytic product. We present the regulation strategy for the isomerization of azobenzene derivatives enabled here, through the controlled adjustment of reactant concentration, and expand the application of SERS technique.

Solvent-controlled plasmon-assisted surface catalysis reaction of 4-aminothiophenol dimerizing to p,p'-dimercaptoazobenzene on Ag nanoparticles

A large number of literatures have investigated the selective photocatalytic reaction of 4-aminothiophenol (PATP) to p,p'-dimercaptoazobenzene (DMAB). Most of them mainly study the contribution of substrate, excitation wavelength, exposure time, pH and added cations to plasmon-assisted surface catalytic reactions. However, we mainly study focuses on the effects of solvents on the dimerization of PATP to DMAB under the action of Ag nanoparticles (NPs). In experiments, a variety of diols was selected as solvents for the probe molecule PATP, and power-dependent SERS spectra were obtained at an excitation wavelength of 532 nm. From the laser-dependent SERS spectrum, we found that the characteristic peak enhancement effect of the product DMAB in different solvents is significantly different. That is, different solvents could regulate the rate at which DMAB is produced from PATP. Based on the experimental results, we further explored how different diol solvents regulate the response of PATP to DMAB. Our conclusion is that the solvent in the system can quickly capture the hot electrons generated by the decay of the plasmon, so that the remaining holes can oxidize PATP to form DMAB. The ability to trap hot electrons is different due to the difference in the position of the functional groups in the solvent, so that the photocatalytic reaction rate of the hole-oxidized PATP is different. The ability to capture electrons varies depending on the position of the functional groups in the solvent, so the oxidation rate of the photocatalytic reaction is also different. This work not only deepens our understanding of the mechanism of hole-driven surface catalysis oxidation reaction, but also provides a convenient method for regulating the rate of catalytic oxidation.

para-Aminothiophenol Radical Reaction-Functionalized Gold Nanoprobe for One-to-All Detection of Five Reactive Oxygen Species In Vivo

Five major reactive oxygen species (ROS) are generated in diseases including H₂O₂, ^•OH, O₂^•-, ROO^•, and ¹O₂. Simultaneous detection of the five ROS with a single probe is crucial for a comprehensive understanding of the development and progression of many diseases, such as cancer and inflammatory diseases. However, currently reported detection systems are limited by targeting one ROS with one probe. This one-to-one detection mode may fail to sufficiently unveil the diseased state. In this study, we achieved simultaneous detection of all the five ROS with one probe (i.e., one-to-all detection), by designing a novel para-aminothiophenol (PATP) and hemin-decorated gold (Au/PATP/Hemin) nanoprobe. The design is principled by our discovery that PATP can react with ^•OH, O₂^•-, ROO^•, and ¹O₂ by a radical oxidative coupling mechanism to form 4,4'-dimercaptoazobenzene (DMAB). The DMAB then elicited strong characteristic surface-enhanced Raman scattering (SERS) peaks at 1142, 1386, and 1432 cm^-1; which in turn enables direct detection of ^•OH, O₂^•-, ROO^•, and ¹O₂ and indirect detection of H₂O₂ by hemin-catalyzed fenton reaction to convert H₂O₂ into ^•OH. In two representative ROS-elevated mice models of tumors and allergic dermatitis, the Au/PATP/Hemin nanoprobe demonstrated its robust performance of monitoring tumor development and inflammation progression in a highly sensitive and quantitative manner.

Exciton-plasmon hybrids for surface catalysis detected by SERS

Surface plasmons (SPs), in which the free electrons are collectively excited on the metal surface, have been successfully used in chemical analysis and signal detection. Generally, SPs possess two types of decay channels. SPs decay either nonradiatively via the generation of hot electrons or radiatively through re-emitted photons, which can trigger surface chemical reactions when the molecules are adsorbed on the surface of metal nanoparticles. An excitation light with a special wavelength is irradiated on the surface of the plasmonic nanostructure, the strong coupling interaction between electrons and light will then occur on this, and this is followed by the development of a series of unique properties. 2D materials have been a hot topic of research for more than a decade, since graphene was found in 2004. Recently, the combination of graphene with metal NPs has been shown to possess many supernormal advantages, such as high stability and catalytic activity, which have been successfully applied in plasmon-exciton co-driven chemical reactions.

Diazotization-Coupling Reaction-Based Determination of Tyrosine in Urine Using Ag Nanocubes by Surface-Enhanced Raman Spectroscopy

A novel, simple, and highly sensitive method was developed to detect the concentration of tyrosine-derived azo dye indirectly using silver nanocubes (AgNCs) as a substrate on a super-hydrophobic silver film by surface-enhanced Raman spectroscopy (SERS). Diazotization-coupling reaction occurred between diazonium ions and the phenolic tyrosine, resulting in three new typical peaks in the SERS spectrum of the azo dye that was formed on the AgNCs, indicating strong SERS activity. Subsequently, the limit of detection of this approach was as low as 10^-12 M for tyrosine. Moreover, the SERS intensities of the three typical SERS signals of the analyte were linearly correlated with the logarithm of concentration of the Tyrosine. The proposed method shows great potential for tyrosine detection in the urine samples of normal humans.

Surface-enhanced Raman spectroscopic detection of molecular chemo- and plasmo-catalysis on noble metal nanoparticles

The in situ detection of reactions catalyzed by metal NPs is challenging because the underlying chemical transformations occur at interfaces. Surface-enhanced Raman scattering (SERS), a surface-selective, sensitive and label-free vibrational spectroscopic technique, is ideally suited for monitoring of heterogeneous catalysis with high chemical specificity. A major limitation in the past, however, was that small, catalytically active metal NPs do not exhibit the high plasmonic activity required for SERS. This feature article focuses on the design, synthesis and use of bifunctional NPs with both catalytic and plasmonic activity for in situ SERS detection of reactions catalyzed by metal NPs. We focus on model reactions induced by chemical reducing agents such as hydride or molecular hydrogen as well as on plasmon-induced photo-catalysis including both photo-oxidation and photo-reduction. Finally, we highlight the concept of photo-recycling on halide-containing silver surfaces for unprecedented multi-electron reduction chemistry.

Gold Superparticles Functionalized with Azobenzene Derivatives: SERS Nanotags with Strong Signals

The surface-enhanced Raman spectroscopy (SERS) nanotag was proposed as a substitute for fluorescent dye for imaging and biosensors several decades ago. However, its weak signal and poor reproducibility has hindered its application. Here, we report a new strategy to form Au superparticles (AuSPs) with high SERS enhancement via one-pot formation and self-assembly of Au nanoparticles (NPs). An azobenzene-carrying Raman reporter was synthesized to exhibit a large Raman cross-section and multiple bands. The self-assembly of the Raman reporter on AuSPs generated SERS nanotags with intense signals. A Raman reporter carrying boronic acid and azobenzene groups displayed six distinctive bands. Its corresponding SERS nanotag demonstrated a high sensing ability toward glycoprotein through aggregation-induced SERS enhancement or as a substitute for labeled antibodies in an immunoassay of the glycoprotein.

Janus PEGylated gold nanoparticles: a robust colorimetric probe for sensing nitrite ions in complex samples

We presented a Janus PEGylated AuNP probe where PEGs and recognition ligands (e.g., 4-aminobenzenethiol, 4-ABT) were asymmetrically functionalized on an AuNP. With this design, the probes showed high colloidal stability, signal robustness, specificity, and sufficient sensitivity in the determination of NO₂^- in various complex samples.

Chemical Sensing on a Single SERS Particle

We report a new chemical sensing platform on a single surface-enhanced Raman scattering (SERS) particle. A cabbage-like Au microparticle (CLMP) with high SERS enhancement was applied as an ultrasensitive SERS substrate. A new Raman reporter bis[4,4'-[dithiodiphenyl azo-phenol] (DTDPAP) was synthesized to display multiple fingerprints and high reactivity toward sodium dithionite. The reaction of DTDPAP with sodium dithionite was in situ monitored by SERS on a single CLMP. The DTDPAP fingerprint change is dependent on the sodium dithionite concentration, providing a simple and sensitive method for sodium dithionite profiling.

Observing the dynamic “hot spots” on two-dimensional Au nanoparticles monolayer film

Interparticle spacing was controlled by evaporating water on 2D Au nanoparticles arrays.

Recent advances in nanomaterials for water protection and monitoring

Nanomaterials (NMs) for adsorption, catalysis, separation, and disinfection are scrutinized. NMs-based sensor technologies and environmental transformations of NMs are highlighted.

Plasmon induced polymerization using a TERS approach: a platform for nanostructured 2D/1D material production

Plasmon-induced chemical reactions have recently attracted great attention as a promising method for high efficiency light-energy conversion and proved to be useful in a wealth of different domains of chemistry and physics. One of the interesting and, so far, less explored avenues of such reactions is their potential for efficient, highly localized and controlled polymer production. Here, we present the first example of a localized, directed plasmon catalyzed polymerization process of a self-assembled monolayer on both silver and gold surfaces monitored by surface- and tip-enhanced Raman spectroscopy (SERS and TERS). As a proof-of-concept, a bi-functionalized dibenzo(1,2)dithiine-3,8-diamine (D3ATP) molecule that undergoes a well-known plasmon-induced coupling via the amino group into an azo group has been used. Initial dimerization is demonstrated using established marker bands associated with the formation of the azo group. A subsequent indicator for a polymerization reaction, the appearance of a new characteristic band, is monitored by time-dependent SERS and TERS experiments. We demonstrate that the dimerization reaction and hence, the subsequent polymerization, can be induced by a plasmonic feature, e.g. a TERS tip, at specific nanoscale locations and, at a much larger micron scale, by continuously scanning the plasmonic probe. The presented results provide the basis for designing further plasmonic catalysis experiments in general, and offer a new platform for producing ultra-thin polymer films with a defined structural dimension.

A new strategy for effective distance regulation of the surface plasmon assisted coupling reaction of p-nitrothiophenol to p,p′-dimercaptoazobenzene

Plasmon assisted reactions on a metal surface occur through a different mechanism compared to traditional reaction conditions.

A Boronate Affinity-Assisted SERS Tag Equipped with a Sandwich System for Detection of Glycated Hemoglobin in the Hemolysate of Human Erythrocytes

Phenylboronic acid-functionalized, Ag shell-coated, magnetic, monodisperse polymethacrylate microspheres equipped with a glycoprotein-sensitive sandwich system were proposed as a surface-enhanced Raman scattering (SERS) substrate for quantitative determination of glycated hemoglobin (HbA1c). The magnetization of the SERS tag and the formation of the Ag shell on the magnetic support were achieved using the bifunctional reactivity of newly synthesized polymethacrylate microspheres. The hemolysate of human red blood cells containing both HbA1c and nonglycated hemoglobin was used for determination of HbA1c. The working principle of the proposed SERS tag is based on the immobilization of HbA1c by cyclic boronate ester formation between glycosyl residues of HbA1c and boronic acid groups of magnetic polymethacrylate microspheres and the binding of p-aminothiophenol (PATP)-functionalized Ag nanoparticles (Ag NPs) carrying another boronic acid ligand via cyclic boronate ester formation via unused glycosyl groups of bound HbA1c. Then, in situ formation of a Raman reporter, 4,4'-dimercaptoazobenzene from PATP under 785 nm laser irradiation allowed for the quantification of HbA1c bound onto the magnetic SERS tag, which was proportional to the HbA1c concentration in the hemolysate of human erythrocytes. The sandwich system provided a significant enhancement in the SERS signal intensity due to the plasmon coupling between Ag NPs and Ag shell-coated magnetic microspheres, and low HbA1c concentrations down to 50 ng/mL could be detected. The calibration curve obtained with a high correlation coefficient between the SERS signal intensity and HbA1c level showed the usability of the SERS protocol for the determination of the HbA1c level in any person.

Reliable SERS detection of nitrite based on pH and laser irradiance-dependent diazotization through a convenient sampling micro-chamber

A SERS-enabled micro-chamber was constructed for reliable and pretreatment-free detection of NO₂⁻based on a pH and laser irradiance-dependent diazotization.