Detection of nitrite with a surface-enhanced Raman scattering sensor based on silver nanopyramid array

Deep Learning-Powered Colloidal Digital SERS for Precise Monitoring of Cell Culture Media

Maintaining consistent quality in biopharmaceutical manufacturing is essential for producing high-quality complex biologics. Yet, current process analytical technologies (PAT) struggle to achieve rapid and highly accurate monitoring of small molecule critical process parameters and critical quality attributes. While Raman spectroscopy holds great promise as a highly sensitive and specific bioanalytical tool for PAT applications, its conventional implementation, surface-enhanced Raman spectroscopy (SERS), is constrained by considerable temporal and spatial intensity fluctuations, limiting the achievable reproducibility and reliability. Herein, we introduce a deep learning-powered colloidal digital SERS platform to address these limitations. Rather than addressing the intensity fluctuations, the approach leverages their very stochastic nature, arising from highly dynamic analyte-nanoparticle interactions. By converting the temporally fluctuating SERS intensities into digital binary "ON/OFF" signals using a predefined intensity threshold by analyzing the characteristic SERS peak, this approach enables digital visualization of single-molecule events and significantly reduces false positives and background interferences. By further integrating colloidal digital SERS with deep learning, the applicability of this platform is significantly expanded and enables detection of a broad range of analytes, unlimited by the lack of characteristic SERS peaks for certain analytes. We further implement this approach for studying AMBIC 1.1, a chemically-defined, serum-free complete media for mammalian cell culture. The obtained highly accurate and reproducible results demonstrate the unique capabilities of this platform for rapid and precise cell culture media monitoring, paving the way for its widespread adoption and scaling up as a new PAT tool in biopharmaceutical manufacturing and biomedical diagnostics.

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.

Nanofibers decorated with high-entropy alloy particles for the detection of nitrites

Excessive residues of nitrite can pose a serious threat to human health, making the establishment of an efficient and effective electrochemical sensor for nitrite detection highly necessary. Herein, we report on a sensor based on nitrogen-doped carbon nanofibers, with FeCoNiCuAl high-entropy alloy (HEAs) nanoparticles in situ grown on the carbon fibers through a confinement effect. The FeCoNiCuAl/CNF sensor is capable of electrochemically detecting nitrite using both differential pulse voltammetry (DPV) and amperometric (I-t) methods. The DPV detection offers a linear range of 0.1-5000 μM and 5000-18 000 μM, with sensitivities of 150.6 μA mM-1 cm-2 and 80.1 μA mM-1 cm-2 and a detection limit of 0.023 μM (S/N = 3). The I-t detection covers a range of 1-10 000 μM, with a sensitivity of 337.84 μA mM-1 cm-2 and a detection limit of 0.12 μM. Moreover, the sensor exhibits excellent anti-interference properties, stability, and reproducibility, providing feasibility for nitrite detection in real-world environments.

Reaction-Based SERS Probes for the Detection of Raman-Inactive Species

Surface-enhanced Raman spectroscopy (SERS) has the advantages of high sensitivity, low water interference, narrow spectral peaks for multicomponent analysis, and rich molecular fingerprint information, presenting great potential to be a robust analytical technology. However, a key issue is the unavailability in directly detecting Raman-inactive species with a small Raman scattering cross-section. Current research has addressed this issue by using specific chemical reactions to induce significant characteristic changes in SERS signals, enabling the sensitive and selective detection of Raman-inactive species. This reaction-activated SERS sensing strategy provides a clever approach to the precise determination of Raman-inactive species. In this review, we have first summarized the design principles and types of reaction-based SERS probes. Furthermore, we have examined the enormous potential of reaction-based SERS probes in the detection of bioactive species, environmental pollutants, and food contaminants. Finally, we have discussed in depth the challenges and prospects of reaction-based SERS probes on stability, reliability, and intelligence. The review is aimed to inspire a more advanced design of reaction-based SERS probes, thus further facilitating their extensive applications in SERS analysis.

Multiplexed SERS Detection of Serum Cardiac Markers Using Plasmonic Metasurfaces

Surface-enhanced Raman spectroscopy (SERS) possesses exquisite molecular-specific properties with single-molecule sensitivity. Yet, translation of SERS into a quantitative analysis technique remains elusive owing to considerable fluctuation of the SERS intensity, which can be ascribed to the SERS uncertainty principle, a tradeoff between "reproducibility" and "enhancement". To provide a potential solution, herein, an integrated multiplexed SERS biosensing strategy is proposed, which features two distinct advantages. First, a subwavelength-structured plasmonic metasurface consisting of alternately stacked metal-dielectric pyramidal meta-atoms is fabricated and could provide simultaneously enhanced electric and magnetic fields to enable spatially extended and weakly wavelength-dependent SERS. Second, nanomechanical perturbations are harnessed to transduce signals in the form of SERS frequency shifts, which are not directly affected by the SERS uncertainty principle. By also employing 3D printing methods, a proof-of-concept study of multiplexed detection of a panel of serum cardiac biomarkers for acute myocardial infarction is provided. Success in the development of both the electric and magnetic fields-active plasmonic metasurfaces could transform future designs of SERS substrates with newly endowed functionalities, and frequency shift-based SERS multiplexing could open new opportunities to develop innovative quantitative optical techniques for applications in chemistry, biology, and medicine.

Interfacial Electronic Interactions Promoted Activation for Nitrate Electroreduction to Ammonia over Ag-Modified Co3O4

Electrocatalytic nitrate (NO3 -) reduction to ammonia (NRA) offers a promising pathway for ammonia synthesis. The interfacial electronic interactions (IEIs) can regulate the physicochemical capabilities of catalysts in electrochemical applications, while the impact of IEIs on electrocatalytic NRA remains largely unexplored in current literature. In this study, the high-efficiency electrode Ag-modified Co3O4 (Ag1.5Co/CC) is prepared for NRA in neutral media, exhibiting an impressive nitrate conversion rate of 96.86 %, ammonia Faradaic efficiency of 96.11 %, and ammonia selectivity of ~100 %. Notably, the intrinsic activity of Ag1.5Co/CC is ~81 times that of Ag nanoparticles (Ag/CC). Multiple characterizations and theoretical computations confirm the presence of IEIs between Ag and Co3O4, which stabilize the CoO6 octahedrons within Co3O4 and significantly promote the adsorption of reactants (NO3 -) as well as intermediates (NO2 - and NO), while suppressing the Heyrovsky step, thereby improving nitrate electroreduction efficiency. Furthermore, our findings reveal a synergistic effect between different active sites that enables tandem catalysis for NRA: NO3 - reduction to NO2 - predominantly occurs at Ag sites while NO2 - tends to hydrogenate to ammonia at Co sites. This study offers valuable insights for the development of high-performance NRA electrocatalysts.

Development of an accurate hand-held sensing platform for nitrite detection based on nitrogen-doped carbon dots

As is well known, excessive nitrite can seriously pollute the environment and can harm human health. Although existing methods can be used to determine nitrite content, they still have some drawbacks, such as relatively complicated operation and expensive equipment. Herein, a hand-held sensing platform (HSP) for NO2- determination was developed. First, ammonia-rich nitrogen-doped carbon dots with orange-yellow emission were designed and synthesised, which were suitable as fluorescent probes because of their good optical properties and stability. Then, the HSP based on fluorescence using photoelectric conversion technology was designed and manufactured using three-dimensional printing technology. Under optimum conditions, the voltage (V/V0) of the proposed HSP showed good linearity for NO2- detection in the range of 10-500 μM, with a detection limit of 1.95 μM. This portable sensor showed good stability, accuracy and reliability in detecting actual water and meat samples, which may ensure food safety in practical applications. Moreover, the HSP is compact, portable and easily assembled and is suitable for on-site real-time detection, which shows great application potential and prospects.

Label-free plasmonic spectral profiling of serum DNA

Genetic and epigenetic modifications are linked to the activation of oncogenes and inactivation of tumor suppressor genes. Likewise, the associated molecular alternations can best inform precision medicine for personalized tumor treatment. Therefore, performing characterization of genetic and epigenetic alternations at the molecular level represents a crucial step in early diagnosis and/or therapeutics of cancer. However, the prevailing methods for DNA analysis involve a series of tedious and complicated steps, in which important genetic and epigenetic information could be lost or altered. To provide a potential approach for non-invasive, direct, and efficient DNA analysis, herein, we present a promising strategy for label-free molecular profiling of serum DNA in its pristine form by fusing surface-enhanced Raman spectroscopy with machine learning on a superior plasmonic nanostructured platform. Using DNA methylation and single-point mutation as two case studies, the presented strategy allows a well-balanced sensitive and specific detection of epigenetic and genetic changes at the single-nucleotide level in serum. We envision the presented label-free strategy could serve as a versatile tool for direct molecular profiling in pristine forms of a wide range of biological markers and aid biomedical diagnostics as well as therapeutics.

Plasmonic silver and gold nanoparticles: shape- and structure-modulated plasmonic functionality for point-of-caring sensing, bio-imaging and medical therapy

Silver and gold nanoparticles have found extensive biomedical applications due to their strong localized surface plasmon resonance (LSPR) and intriguing plasmonic properties. This review article focuses on the correlation among particle geometry, plasmon properties and biomedical applications. It discusses how particle shape and size are tailored via controllable synthetic approaches, and how plasmonic properties are tuned by particle shape and size, which are embodied by nanospheres, nanorods, nanocubes, nanocages, nanostars and core-shell composites. This article summarizes the design strategies for the use of silver and gold nanoparticles in plasmon-enhanced fluorescence, surface-enhanced Raman scattering (SERS), electroluminescence, and photoelectrochemistry. It especially discusses how to use plasmonic nanoparticles to construct optical probes including colorimetric, SERS and plasmonic fluorescence probes (labels/reporters). It also demonstrates the employment of Ag and Au nanoparticles in polymer- and paper-based microfluidic devices for point-of-care testing (POCT). In addition, this article highlights how to utilize plasmonic nanoparticles for in vitro and in vivo bio-imaging based on SERS, fluorescence, photoacoustic and dark-field models. Finally, this article shows perspectives in plasmon-enhanced photothermal and photodynamic therapy.

Robust and sensitive determination of nitrites in pickled food by surface-enhanced Raman spectroscopy

Nitrites are ubiquitous in food and pose a serious threat to human health. Therefore, the rapid and accurate determination of nitrite ion concentration in food is a prerequisite for eliminating the damage of nitrites. In this study, a robust, rapid, and sensitive method is proposed for nitrite detection in pickled food, in which Au@Ag nanoparticles are used as a reliable surface-enhanced Raman spectroscopy (SERS) substrate taking advantage of the high enhancement effect of silver and the good stability of gold. Nitrites were anchored to the surface of the SERS substrate by bridging with 4-aminophenylthiophenol (PATP). With Raman scattering cross-section amplification and internal calibration by PATP, a satisfactory linear relationship (R2 = 0.987) was established for nitrite detection in the concentration range of 5.00-100.00 μM, and the limit of detection (LOD) was 0.17 μM. This SERS-based method demonstrated high selectivity, good precision (RSD < 7.00 %), and satisfying recovery rates (101.42-107.35 %) in real samples, thus improving the determination method for nitrites. Therefore, this method has application potential in food safety and supervision.

A simple and efficient alginate hydrogel combined with surface-enhanced Raman spectroscopy for quantitative analysis of sodium nitrite in meat products

Sodium nitrite is a commonly used preservative and color protectant in the food industry. Conventional analytical methods are highly susceptible to food matrix interference, time-consuming and costly. In this study, the ion cross-linking method was employed to prepare alginate hydrogel substrates, and phenosafranin was chosen as a single-molecule probe to analyze sodium nitrite. Our investigation centered on elucidating the effects of alginate and cross-linking ion concentrations on Raman signal characteristics. The optimal Raman response was observed in the precursor solution with 1% sodium alginate and 0.1 mol L-1 cross-linking ions. The relative standard deviations (RSDs) of the feature peaks from the three substrate batches ranged from 1.22% to 16.30%, attesting the robustness and consistency of the substrates. The signal reduction of the substrates after a four-week storage period remained below 10%, indicating that the substrates had good reproducibility and stability. The limits of detection (LODs) for sodium nitrite in extracts from cured meat, luncheon meat, and sliced ham were determined to range from 3.75 mg kg-1 to 8.11 mg kg-1, with low interference from the food matrix. The support vector machine algorithm was utilized to train and predict the data, which proved to be more accurate (98.6%-99.8% recovery) than the traditional linear regression model (81.9%-112.7% recovery) in predicting the spiked samples. The application of hydrogel-based surface-enhanced Raman spectroscopy (SERS) substrates for nitrite detection in food, combined with machine learning for regression prediction in data processing, collectively augmented the potential of SERS technology in the field of food analysis.

Quantitative Detection of Thyroid-Stimulating Hormone in Patient Samples with a Nanomechanical Single-Antibody Spectro-Immunoassay

Functional disorders of the thyroid remain a global challenge and have profound impacts on human health. Serving as the barometer for thyroid function, thyroid-stimulating hormone (TSH) is considered the single most useful test of thyroid function. However, the prevailing TSH immunoassays rely on two types of antibodies in a sandwich format. The requirement of repeated incubation and washing further complicates the issue, making it unable to meet the requirements of the shifting public health landscape that demands rapid, sensitive, and low-cost TSH tests. Herein, a systematic study is performed to investigate the clinical translational potential of a single antibody-based biosensing platform for the TSH test. The biosensing platform leverages Raman spectral variations induced by the interaction between a TSH antigen and a Raman molecule-conjugated TSH antibody. In conjunction with machine learning, it allows TSH concentrations in various patient samples to be predicted with high accuracy and precision, which is robust against substrate-to-substrate, intra-substrate, and day-to-day variations. It is envisioned that the simplicity and generalizability of this single-antibody immunoassay coupled with the demonstrated performance in patient samples pave the way for it to be widely applied in clinical settings for low-cost detection of hormones, other molecular biomarkers, DNA, RNA, and pathogens.

Quantum Plexcitonic Sensing

While fundamental to quantum sensing, quantum state control has been traditionally limited to extreme conditions. This restricts the impact of the practical implementation of quantum sensing on a broad range of physical measurements. Plexcitons, however, provide a promising path under ambient conditions toward quantum state control and thus quantum sensing, owing to their origin from strong plasmon-exciton coupling. Herein, we harness plexcitons to demonstrate quantum plexcitonic sensing by strongly coupling excitonic particles to a plasmonic hyperbolic metasurface. As compared to classical sensing in the weak-coupling regime, our model of quantum plexcitonic sensing performs at a level that is ∼40 times more sensitive. Noise-modulated sensitivity studies reinforce the quantum advantage over classical sensing, featuring better sensitivity, smaller sensitivity uncertainty, and higher resilience against optical noise. The successful demonstration of quantum plexcitonic sensing opens the door for a variety of physical, chemical, and biological measurements by leveraging strongly coupled plasmon-exciton systems.

Ratiometric colorimetric detection of nitrite using CoMnO3 nanofibers as an oxidase-like enzyme to induce diazotization reaction

Nitrite is a typical food additive and preservative used in the food industry, which has attracted considerable attention due to its severe adverse effects on human health. Herein, a sensitive and highly selective ratiometric colorimetric sensing platform for the detection of nitrite was created based on a polymetallic oxide nanozyme, CoMnO3 nanofibers (CMO) catalysis integrated with the particular diazotization reaction. The nanozyme has superior oxidase-like activity (Km was 0.105 mM and Vmax was 63.7 × 10-8 M S-1) and could catalyze the oxidation of 3,3',5,5'-tetramethylbenzidine (TMB) to oxidized TMB (oxTMB), as CMO could achieve the conversion of oxygen in the solution to superoxide anion (O2˙-). In addition, it is interesting to note that oxTMB can be diazotized in the presence of nitrite under acidic conditions, causing a shift in the ratio of nitrite concentration to the absorbance peaks at 450 and 652 nm (A450/A652). The ratio of A450/A652 exhibited a positive linear relationship with the concentration of nitrite within the concentration range of 0.2-200 μM, with a detection limit of 0.094 μM. Simultaneously, this method was also successful in quantifying the nitrite produced by brined and pickled foods and the dynamic tracking of the nitrite levels in various types of dishes. The analysis method not only offers dual-signal ratio sensing with high sensitivity but also holds the benefit of outstanding selectivity for the use of the particular reaction, which has a wide range of application prospects in food safety management.

Non-uniform droplet deposition on femtosecond laser patterned superhydrophobic/superhydrophilic SERS substrates for high-sensitive detection

Surface-enhanced Raman scattering (SERS) sensors combined with superhydrophobic/superhydrophilic (SH/SHL) surfaces have shown the ability to detect ultra-low concentrations. In this study, femtosecond laser fabricated hybrid SH/SHL surfaces with designed patterns are successfully applied to improve the SERS performances. The shape of SHL patterns can be regulated to determine the droplet evaporation process and deposition characteristics. The experimental results show that the uneven droplet evaporation along the edges of non-circular SHL patterns facilitates the enrichment of analyte molecules, thereby enhancing the SERS performance. The highly identifiable corners of SHL patterns are beneficial for capturing the enrichment area during Raman tests. The optimized 3-pointed star SH/SHL SERS substrate shows a detection limit concentration as low as 10^-15 M by using only 5 µL R6G solutions, corresponding to an enhancement factor of 9.73 × 10¹¹. Meanwhile, a relative standard deviation of 8.20% can be achieved at a concentration of 10^-7 M. The research results suggest that the SH/SHL surfaces with designed patterns could be a practical approach in ultratrace molecular detections.

Pyramidal hyperbolic metasurfaces enhance spontaneous emission of nitrogen-vacancy centers in nanodiamond

Nitrogen-vacancy (NV) centers in nanodiamond hold great promise for creating superior biological labels and quantum sensing methods. Yet, inefficient photon generation and extraction from excited NV centers restricts the achievable sensitivity and temporal resolution. Herein, we report an entirely complementary route featuring pyramidal hyperbolic metasurface to modify the spontaneous emission of NV centers. Fabricated using nanosphere lithography, the metasurface consists of alternatively stacked silica-silver thin films configured in a pyramidal fashion, and supports both spectrally broadband Purcell enhancement and spatially extended intense local fields owing to the hyperbolic dispersion and plasmonic coupling. The enhanced photophysical properties are manifested as a simultaneous amplification to the spontaneous decay rate and emission intensity of NV centers. We envision the reported pyramidal metasurface could serve as a versatile platform for creating chip-based ultrafast single-photon sources and spin-enhanced quantum biosensing strategies, as well as aiding in further fundamental understanding of photoexcited species in condensed phases.

Plasmonic surface-enhanced Raman scattering nano-substrates for detection of anionic environmental contaminants: Current progress and future perspectives

Surface-enhanced Raman scattering spectroscopy (SERS) is a powerful technique of vibrational spectroscopy based on the inelastic scattering of incident photons by molecular species. It has unique properties such as ultra-sensitivity, selectivity, non-destructivity, speed, and fingerprinting properties for analytical and sensing applications. This enables SERS to be widely used in real-world sample analysis and basic plasmonic mechanistic studies. However, the desirable properties of SERS are compromised by the high cost and low reproducibility of the signals. The development of multifunctional, stable and reusable nano-engineered SERS substrates is a viable solution to circumvent these drawbacks. Recently, plasmonic SERS active nano-substrates with various morphologies have attracted the attention of researchers due to promising properties such as the formation of dense hot spots, additional stability, tunable and controlled morphology, and surface functionalization. This comprehensive review focused on the current advances in the field of SERS active nanosubstrates suitable for the detection and quantification of anionic environmental pollutants. The common fabrication methods, including the techniques for morphological adjustments and surface modification, substrate categories, and the design of nanotechnologically fabricated plasmonic SERS substrates for anion detection are systematically presented. Here, the need for the design, synthesis, and functionalization of SERS nano-substrates for anions of great environmental importance is explained in detail. In addition, the broad categories of SERS nano-substrates, namely colloid-based SERS substrates and solid-support SERS substrates are discussed. Moreover, a brief discussion of SERS detection of certain anionic pollutants in the environment is presented. Finally, the prospects in the fabrication and commercialization of pilot-scale handheld SERS sensors and the construction of smart nanosubstrates integrated with novel amplifying materials for the detection of anions of environmental and health concern are proposed.

A fluorescent carbon dots synthesized at room temperature for automatic determination of nitrite in Sichuan pickles

In this paper, highly fluorescent carbon dots were synthesized from sodium ascorbate and polyethyleneimine at room temperature (R-CDs). The proposed green synthesis method was energy-saving, environmentally friendly and easy online. R-CDs exhibit an optimal emission peak of 490 nm under excitation at 380 nm with a quantum yield of 32 %. R-CDs morphology, composition, and properties were characterized using TEM, FTIR, XPS, UV-vis and fluorescence spectroscopy. The study revealed that nitrite quenched the fluorescence of R-CDs under acidic conditions. Subsequently, this discovered reaction of R-CDs and nitrite was combined with flow-injection technology, and a simple, precise and automatic fluorescence strategy for nitrite determination was accomplished. The response to nitrite was linear in 5-300 μg·L^-1 concentration range and the limit of detection was 2.85 μg·L^-1 (3.3 S/k). This method was applied to nitrite determination in Sichuan pickles during the pickling process and results were consistent with the standard method, demonstrating its feasibility in practical applications.

Partnered Excited-State Intermolecular Proton Transfer Fluorescence (P-ESIPT) Signaling for Nitrate Sensing and High-Resolution Cell-Imaging

Nitrite (NO₂⁻) is a common pollutant and is widely present in the environment and in human bodies. The development of a rapid and accurate method for NO₂⁻ detection is always a very important task. Herein, we synthesized a partnered excited-state intermolecular proton transfer (ESIPT) fluorophore using the “multi-component one pot” method, and used this as a probe (ESIPT-F) for sensing NO₂⁻. ESIPT-F exhibited bimodal emission in different solvents because of the solvent-mediated ESIPT reaction. The addition of NO₂⁻ caused an obvious change in colors and tautomeric fluorescence due to the graft of NO₂⁻ into the ESIPT-F molecules. From this basis, highly sensitive and selective analysis of NO₂⁻ was developed using tautomeric emission signaling, achieving sensitive detection of NO₂⁻ in the concentration range of 0~45 mM with a detection limit of 12.5 nM. More importantly, ESIPT-F showed the ability to anchor proteins and resulted in a recognition-driven “on-off” ESIPT process, enabling it to become a powerful tool for fluorescence imaging of proteins or protein-based subcellular organelles. MTT experimental results revealed that ESIPT-F is low cytotoxic and has good membrane permeability to cells. Thus, ESIPT-F was further employed to image the tunneling nanotube in vitro HEC-1A cells, displaying high-resolution performance.

Fluorine and nitrogen co-doped near-infrared carbon dots for fluorescence "on-off-on" determination of nitrite

A fluorescence "on-off-on" strategy was established for the determination of nitrite in aqueous solution based on fluorine and nitrogen co-doped near-infrared carbon dots (NIR-CDs). NIR-CDs were prepared via one-step hydrothermal method by using N-(4-aminophenyl)-acetamide and 4,5-difluorobenzene-1,2-diamine as precursors. The photoluminescence quantum yield of NIR-CDs reaches to 17.4%, and the optimal emission peak of NIR-CDs is 675 nm under excitation of 530 nm. The Stokes shift of NIR-CDs (145 nm) is higher than that of some CDs with longer emission wavelengths. The red bathophenanthroline disulfonic acid (BPS)-Fe²⁺ complex can quench the fluorescence of NIR-CDs via inner filter effect and static quench modes. Nitrite can oxidize Fe²⁺ to produce Fe³⁺ in acidic environment, resulting in not only the formation of colorless and unstable BPS-Fe³⁺ complex but also the fluorescence recovery of NIR-CDs. This fluorescence "on-off-on" phenomenon also comes with the color variation of the mixture, resulting in both the fluorescence and the visual determination of nitrite. Under optimal conditions, this assay exhibits a good linear range from 1 to 50 μM and a low detection limit of 0.056 μM for nitrite determination. The method showed good applicability for nitrite determination in soil extract, human urine, and water samples with acceptable results. A convenient fluorescence "on-off-on" strategy for nitrite detection based on fluorine and nitrogen co-doped near-infrared carbon dots (NIR-CDs) and bathophenanthroline disulfonic acid (BPS)-Fe²⁺ complex was innovatively established. This probe showed a low detection limit of 0.056 μM for nitrite in authentic samples, which offered a new sight for fluorescent and visual detection of nitrite in environmental protection and human health areas.

A Dual-Modal Single-Antibody Plasmonic Spectro-Immunoassay for Detection of Small Molecules

Small molecules play a pivotal role in regulating physiological processes and serve as biomarkers to uncover pathological conditions and the effects of therapeutic treatments. However, it remains a significant challenge to detect small molecules given the size as compared to macromolecules. Recently, the newly emerging plasmonic immunoassays based on surface-enhanced Raman scattering (SERS) offer great promise to deliver extraordinary sensitivity. Nevertheless, they are limited by the intrinsic SERS intensity fluctuations associated with the SERS uncertainty principle. The single transducer that relies on the intensity change is also prone to false signals. Additionally, the prevailing sandwich immunoassay format proves less effective towards detecting small molecules. To circumvent these critical issues, a dual-modal single-antibody approach that synergizes both the intensity and shift of the peak-based immunoassay with Raman enhancement, coined as the INSPIRE assay, is developed for small molecules detection. With two independent transduction mechanisms, it allows better prediction of analyte concentration and attenuation of signal artifacts, providing a new and robust strategy for molecular analysis. With a proof-of-concept demonstration for detection of free T4 and testosterone in serum matrix, the authors envision that the INSPIRE assay could be expanded for a wide spectrum of applications in biomedical diagnosis, discovery of new biopharmaceuticals, food safety, and environmental monitoring.

Electrochemical Self-Assembled Gold Nanoparticle SERS Substrate Coupled with Diazotization for Sensitive Detection of Nitrite

The accurate determination of nitrite in food samples is of great significance for ensuring people's health and safety. Herein, a rapid and low-cost detection method was developed for highly sensitive and selective detection of nitrite based on a surface-enhanced Raman scattering (SERS) sensor combined with electrochemical technology and diazo reaction. In this work, a gold nanoparticle (AuNP)/indium tin oxide (ITO) chip as a superior SERS substrate was obtained by electrochemical self-assembled AuNPs on ITO with the advantages of good uniformity, high reproducibility, and long-time stability. The azo compounds generated from the diazotization-coupling reaction between nitrite, 4-aminothiophenol (4-ATP), and N-(1-naphthyl) ethylenediamine dihydrochloride (NED) in acid condition were further assembled on the surface of AuNP/ITO. The detection of nitrite was realized using a portable Raman spectrometer based on the significant SERS enhancement of azo compounds assembled on the AuNP/ITO chip. Many experimental conditions were optimized such as the time of electrochemical self-assembly and the concentration of HAuCl₄. Under the optimal conditions, the designed SERS sensor could detect nitride in a large linear range from 1.0 × 10^-6 to 1.0 × 10^-3 mol L^-1 with a low limit of detection of 0.33 μmol L^-1. Additionally, nitrite in real samples was further analyzed with a recovery of 95.1-109.7%. Therefore, the proposed SERS method has shown potential application in the detection of nitrite in complex food samples.

Visible-light and near-infrared fluorescence and surface-enhanced Raman scattering point-of-care sensing and bio-imaging: a review

This review article deals with the concepts, principles and applications of visible-light and near-infrared (NIR) fluorescence and surface-enhanced Raman scattering (SERS) in in vitro point-of-care testing (POCT) and in vivo bio-imaging. It has discussed how to utilize the biological transparency windows to improve the penetration depth and signal-to-noise ratio, and how to use surface plasmon resonance (SPR) to amplify fluorescence and SERS signals. This article has highlighted some plasmonic fluorescence and SERS probes. It has also reviewed the design strategies of fluorescent and SERS sensors in the detection of metal ions, small molecules, proteins and nucleic acids. Particularly, it has provided perspectives on the integration of fluorescent and SERS sensors into microfluidic chips as lab-on-chips to realize point-of-care testing. It has also discussed the design of active microfluidic devices and non-paper- or paper-based lateral flow assays for in vitro diagnostics. In addition, this article has discussed the strategies to design in vivo NIR fluorescence and SERS bio-imaging platforms for monitoring physiological processes and disease progression in live cells and tissues. Moreover, it has highlighted the applications of POCT and bio-imaging in testing toxins, heavy metals, illicit drugs, cancers, traumatic brain injuries, and infectious diseases such as COVID-19, influenza, HIV and sepsis.

Hexagonal arrays of plasmonic gold nanopyramids on flexible substrates for surface-enhanced Raman scattering

Surface-enhanced Raman spectroscopy (SERS) with pyramidal gold nanostructures increases the signal of Raman active analytes, since hotspots form at the edges and tip of a nanopyramid under illumination. 2D hexagonal arrays of pyramidal nanostructures with a quadratic base are fabricated through cost-effective nanosphere lithography and transferred onto elastomeric polydimethylsiloxane. By making use of the {111} crystal plane of a silicon (100) wafer, an inverted pyramidal array is etched, which serves as the complementary negative for the gold nanostructures. Either a continuous gold thin-film with protruding pyramids or separate isolated nanopyramids are produced. Three basic fabrication strategies are presented. The SERS enhancement is verified by Raman mapping of 4-mercaptobenzoic acid (4-MBA) molecules. Fabrication on a flexible substrate paves the way for future applications on curved surfaces orin situtunable resonances.

Cost-effective large-area Ag nanotube arrays for SERS detections: effects of nanotube geometry

This study demonstrated highly-ordered metallic nanotube arrays (MeNTAs) with a precisely controlled geometric shape to promote surface-enhanced Raman scattering (SERS). Using both simulation and experimental methods, we designed and fabricated MeNTAs with nanotube geometries that possess a large surface area to absorb probe molecules as well as geometric features capable of inducing hot spots for SERS enhancement. The proposed top-down wafer-scale lithographic and sputter-deposition process is a simple and cost-effective approach to the fabrication of 1 mm × 1 mm MeNTA at room temperature. Simulation results of nanotubes with various materials (Au, Ag, and Cu), diameters (100-1500 nm), geometric shapes (circle, equilateral triangle and square) and triangle corner curvatures (ranging from 0 to 300 nm) identified Ag triangles with sharp tips as the geometry best suited to SERS enhancement. The SERS spectra of crystal violet molecules generated from the Ag MeNTAs verified the patterns observed in computational simulations, wherein the effects of MeNTA on SERS decreased with an increase in the size of the nanotubes. Enhancement factor of 1.06 × 10⁹was obtained from our triangular Ag MeNTA, confirming its efficacy as an ultrahigh sensitivity SERS-active substrate.

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.

A novel ratiometric and colorimetric probe for rapid and ultrasensitive detection of nitrite in water based on an Acenaphtho[1,2-d] imidazole derivative

The concentration of nitrite (NO₂^-) ions above allowable limits in water resources and food stuffs is considered hazardous and has been proven to be of great threat to the environment and public health. In this work, an acenaphtho [1,2-d] imidazole derivative (1) as a ratiometric colorimetric probe is developed. UV-Vis experiments demonstrate that the probe 1 shows excellent selectivity toward NO₂^- in the presence of other potential interfering species, a rapid response (20 s) and a low detection limit (100 nM) by a distinct visual color change with a bathochromic shift of 120 nm from colorless to intense yellow. Besides, this probe is further used for the quantification of nitrite ions in environmental water resources such as tap water, underground water, and surface water samples. The high recoveries (96-99% with relative standard deviations (RSD) of <2.0%) make the probe 1 a promising candidate for practical applications in daily life in the detection of nitrite ions.

Chitosan-derived N-doped carbon dots for fluorescent determination of nitrite and bacteria imaging

N-doped carbon dots (N-CDs) were successfully synthesized via simple one-step hydrothermal carbonization using chitosan as carbon and nitrogen sources. The obtained N-CDs contained a variety of functional groups on the NCDs surface, and exhibited excitation-independent behavior and strong blue fluorescence with a relatively higher fluorescence quantum yield (QY = 35%). It also presented excellent water solubility, resistance to pH change, high ion strength and UV irradiation. Since the fluorescence of the N-CDs could be selectively quenched by NO₂^-, they could act as a fluorescent sensor for the determination of NO₂^- in real tap water and lake water samples with a wide linear range (1-500 μM) and low detection limit (0.1 μM). They could also be used for bacterial imaging as multicolor fluorescent probes. The results indicated that N-CDs could be a promising candidate material for biomedical applications.

A highly selective chromogenic probe for the detection of nitrite in food samples

A rapid, sensitive, and highly selective method for determining nitrite in food has been developed. This method is based on the reaction of nitrite with the amino group of 3,3,5,5-tetramethylbenzidine (TMB) to form a diazonium salt, and then the diazonium salt and glucosamine hydrochloride are coupled to each other to form an orange compound. The optimal conditions for maximum color and other analytical parameters were studied. A colorimetric method for nitrite detection has been developed with an outstanding correlation coefficient (R² = 0.9944), a wide linear range (1-75 μM) and 0.73 μM limit of detection (at S/N = 3) for nitrite ions. This method was successfully applied to the determination of nitrite in a variety of foods and gave recoveries in the range between 100.16% and 103.07%, demonstrating that the accuracy, reliability and potential application of this assay for monitoring nitrite in foods.

Fluorescent recognition and selective detection of nitrite ions with carbon quantum dots

The nitrite ion (NO₂^-) is a vital inorganic species that occurs both in natural ecological systems and human bodies. The high concentration of NO₂^- can be harmful for animal and human health. It is important to develop a simple, sensitive, reliable, and economic methodology to precisely monitor NO₂^- in various environmental and biological fields. Thus, a novel nitrite biosensor based on carbon quantum dots (PA-CDs) has been constructed and prepared via a high-efficiency, one-pot hydrothermal route using primary arylamines (PA) such as m-phenylenediamine. The device exhibits bright green fluorescence and a high quantum yield of 20.1% in water. In addition, the PA-CDs also possess two broad linear ranges: 0.05-1.0 μM and 1.0-50 μM with a low detection limit of 7.1 nM. The classical diazo reaction is firstly integrated into the PA-CD system by primary arylamines, which endows the system with high sensitivity and specific selectivity towards nitrite. Importantly, the nanosensor can detect NO₂^- in environmental water and serum samples with high fluorescence recoveries, demonstrating its feasibility in practical applications. This work broadens a new method to fabricate novel nanosensors and provides a prospective application for fluorescent carbon quantum dots (CDs). Graphical abstract.

Redox-derivatization reaction-based rapid and sensitive determination of nitrite using resonance Rayleigh scattering method

It remains a problem for direct detection of small inorganic nitrite ions using resonance Rayleigh scattering (RRS) method based on the direct dye-binding reaction. In the present study, a redox-derivatization reaction taking only 5 min was introduced prior to nitrite detection. In the redox-derivatization reaction, nitrite ions were reduced by excess iodine ions to generate triiodide ions (I₃^-), which were further derivatized with a cationic dye (basic violet 1, BV1) to form the ion associates of I₃^--BV1. Therefore, the RRS signal was significantly enhanced, resulting from the increase of particle size and resonance-enhanced scattering effect. The analytical procedure was performed by just mixing nitrite, oxidant, acid, and dye all-in-one, avoiding the tediousness of a multi-step process or the preparation of nanoparticles. The whole detection process including the redox-derivatization reaction was less than 6 min. The reaction conditions such as concentration of hydrochloric acid, potassium iodide, and BV1, reaction time, and temperature were investigated. Under optimum conditions, the concentration of nitrite was linear with an RRS signal of I₃^--BV1 ion associates at 320 nm in the range of 0.015-1.2 mg/L. The limit of detection (LOD) was calculated to be 3.0 μg/L. The RRS method was applied to the determination of nitrite in real samples such as pork sausage, milk powder, and water with recovery of 95.2-112%. With advantages of rapidness, high sensitivity, and high selectivity, the method indicates potential applicability for detection of nitrite in complex samples. The method also provides an instructive protocol for detection of analytes that generate no/weak RRS enhancement after the direct dye-binding reaction. Graphical abstract.

Development and Application of Aptamer-Based Surface-Enhanced Raman Spectroscopy Sensors in Quantitative Analysis and Biotherapy

Surface-enhanced Raman scattering (SERS) is one of the most special and important Raman techniques. An apparent Raman signal can be observed when the target molecules are absorbed onto the surface of the SERS substrates, especially on the "hot spots" of the substrates. Early research focused on exploring the highly active SERS substrates and their detection applications in label-free SERS technology. However, it is a great challenge to use these label-free SERS sensors for detecting hydrophobic or non-polar molecules, especially in complex systems or at low concentrations. Therefore, antibodies, aptamers, and antimicrobial peptides have been used to effectively improve the target selectivity and meet the analysis requirements. Among these selective elements, aptamers are easy to use for synthesis and modifications, and their stability, affinity and specificity are extremely good; they have been successfully used in a variety of testing areas. The combination of SERS detection technology and aptamer recognition ability not only improved the selection accuracy of target molecules, but also improved the sensitivity of the analysis. Variations of aptamer-based SERS sensors have been developed and have achieved satisfactory results in the analysis of small molecules, pathogenic microorganism, mycotoxins, tumor marker and other functional molecules, as well as in successful photothermal therapy of tumors. Herein, we present the latest advances of the aptamer-based SERS sensors, as well as the assembling sensing platforms and the strategies for signal amplification. Furthermore, the existing problems and potential trends of the aptamer-based SERS sensors are discussed.