Silicon nanohybrid-based SERS chips armed with an internal standard for broad-range, sensitive and reproducible simultaneous quantification of lead(ii) and mercury(ii) in real systems

Nucleic acid-mediated SERS Biosensors: Signal enhancement strategies and applications

Surface Enhanced Raman Spectroscopy (SERS) is a powerful spectroscopic analysis technique applied in various fields due to its high selectivity, ultra-high sensitivity, and non-destructiveness. As natural biological macromolecules, nucleic acids perform a significant role in SERS biosensing. In this review, we first summarize how nucleic acids mediate the signal enhancement of SERS biosensors from three aspects: substrate self-assembly, analyte biorecognition, and molecular amplification. Among them, SERS substrates can be self-assembled by both DNA modification and coordination or electrostatic interactions. In the field of biorecognition, analyte biorecognition based on three nucleic acid recognition elements can enhance SERS signals by regulating the distance of analytes or Raman reporter molecules to the SERS substrate. In addition, nucleic acid-based enzyme and enzyme-free amplification can enhance SERS signals by enlarging the quantity of analytes or its nucleic acid intermediates. Subsequently, multidimensional applications of nucleic acid-mediated SERS signal enhancement in biomedicine, food safety, and environmental monitoring are listed. Finally, the current challenges and future exploration of nucleic acid-mediated SERS signal enhancement are discussed.

Shining a light on environmental science: Recent advances in SERS technology for rapid detection of persistent toxic substances

Persistent toxic substances (PTS) represent a paramount environmental issue in the 21st century. Understanding the concentrations and forms of PTS in the environment is crucial for accurately assessing their environmental health impacts. This article presents a concise overview of the components of PTS, pertinent environmental regulations, and conventional detection methodologies. Additionally, we offer an in-depth review of the principles, development, and practical applications of surface-enhanced Raman scattering (SERS) in environmental monitoring, emphasizing the advancements in detecting trace amounts of PTS in complex environmental matrices. Recent progress in enhancing SERS sensitivity, improving selectivity, and practical implementations are detailed, showcasing innovative materials and methods. Integrating SERS with advanced algorithms are highlighted as pivotal areas for future research.

Factors that Affect Quantification in Surface-Enhanced Raman Scattering

Surface-enhanced Raman scattering (SERS) is an analytical technique capable of detecting trace amounts of specific species. The uniqueness of vibrational signatures is a major advantage of SERS. This combination of sensitivity and specificity has motivated researchers to develop diverse analytical methodologies leveraging SERS. However, even 50 years after its first observation, SERS is still perceived as an unreliable technique for quantification. This perception has precluded the application of SERS in laboratories that rely on consistent quantification (for regulatory purposes, for instance). In this review, we describe some of the aspects that lead to SERS intensity variations and how those challenges were addressed in the 50 years of the technique. The goal is to identify the sources of variations in SERS intensities and then demonstrate that, even with these pitfalls, the technique can be used for quantification when factors such as nature of the substrate, experimental conditions, sample preparation, surface chemistry, and data analysis are carefully considered and tailored for a particular application.

V-Shaped Heterostructure Nanocavities Array with CM and EM Coupled Enhancement for Ultra-Sensitive SERS Substrate

The field of semiconductor surface-enhanced Raman scattering (SERS) substrates has experienced significant advancements, leading to a wide range of applications in several fields. However, the quest for new ultra-sensitive semiconductor SERS materials is still of utmost importance. In this regard, an efficient and novel substrate, F4TCNQ/MoS2 heterostructure is introduced, assisted by V-shaped aluminum anodic oxide (AAO) nanocavities with different depths. Utilizing the efficient charge transfer of organic/inorganic semiconducting heterostructure and the photoconfinement capability of the nanocavity structure of the AAO nanotemplate, excellent stability, fast sensing, enhanced Raman, and photodegradation activities are achieved. Due to its unique 3D structure, the optimized F4TCNQ/MoS2/AAO with 1500 nm depth achieves ultra-high sensitivity detection of 9.0×10-16 M for conventional probe molecules. Furthermore, precise detection of water contaminants is observed for the first time with a V-shaped heterostructure due to combined organic/inorganic features that differ significantly from conventional MoS2 structures or other metal/inorganic or inorganic/inorganic semiconductors. This research presents a novel and versatile strategy for SERS and demonstrates its diverse potential performance in practical applications.

Chip-Scale Aptamer Sandwich Assay Using Optical Waveguide-Assisted Surface-Enhanced Raman Spectroscopy

Chip-scale optical waveguide-assisted surface-enhanced Raman spectroscopy (SERS) that used nanoparticles (NPs) was demonstrated. The Raman signals from Raman reporter (RR) molecules on NPs can be efficiently excited by the waveguide evanescent field when the molecules are in proximity to the waveguide surface. The Raman signal was enhanced by plasmon resonance due to the NPs close to the waveguide surface. The optical waveguide mode and the NP-induced field enhancement were calculated using a finite difference method (FDM). The sensing performance of the waveguide-assisted SERS device was experimentally characterized by measuring the Raman scattering from various RRs, including 4-mercaptobenzoic acid (4-MBA), 5,5'-dithio-bis-(2-nitrobenzoic acid) (DTNB), and malachite green isothiocyanate (MGITC). The observed Raman spectral features were identified and assigned to the complex vibrational modes associated with different reporters. A low detection limit of 1 nM was achieved. In addition, the device sensing method was applied to the detection of the biomarker cardiac troponin I (cTnI) using an aptamer sandwich assay immobilized on the device surface. Overall, the optical waveguides integrated with SERS show a miniaturized sensing platform for the detection of small molecules and large proteins, potentially enabling multiplexed detection for clinically relevant applications.

Development and Application of Surface-Enhanced Raman Scattering (SERS)

Since the discovery of the phenomenon of surface-enhanced Raman scattering (SERS), it has gradually become an important tool for the analysis of material compositions and structures. The applications of SERS have been expanded from the fields of environmental and materials science to biomedicine due to the extremely high sensitivity and non-destructiveness of SERS-based analytical technology that even allows single-molecule detection. This article provides a comprehensive overview of the surface-enhanced Raman scattering (SERS) phenomenon. The content is divided into several main sections: basic principles and the significance of Raman spectroscopy; historical advancements and technological progress in SERS; and various practical applications across different fields. We also discuss how electromagnetic fields contribute to the SERS effect, the role of chemical interactions in enhancing Raman signals, a modeling and computational approaches to understand and predict SERS effects.

A self-calibrating flexible SERS substrate incorporating PB@Au assemblies for reliable and reproducible detection

The precise quantitative analysis using surface-enhanced Raman spectroscopy (SERS) in an uncontrollable environment still faces a significant obstacle due to the poor reproducibility of Raman signals. Herein, we propose a facile method to fabricate a self-calibrating substrate based on a flexible polyvinyl alcohol (PVA) film comprising assemblies of Prussian blue (PB) and Au NPs (PB@Au) for reliable detection. PB cores were coated with an Au shell through simple electrostatic interaction, forming core-shell nanostructure PB@Au assemblies within the PVA film. The outer Au layer provided identical trends in enhancement for both the PB core and neighboring targets while PB cores served as an internal standard (IS) to correct signal fluctuations. The prevention of competitive adsorption on the metal surface between targets and ISs was achieved. The proposed PVA/PB@Au film exhibited enhanced stability of Raman signals after IS correction, resulting in improved spot-to-spot and batch-to-batch reproducibility with significantly reduced standard deviation (RSD) values from 11.42% and 25.02% to 4.43% and 9.39%, respectively. Simultaneously, a higher accuracy in the quantitative analysis of 4-mercaptobenzoic acid (4-MBA) and malachite green (MG) was achieved with fitting coefficient (R2) values improving from 0.9675 and 0.9418 to 0.9974 and 0.9832, respectively. Moreover, the PVA/PB@Au film was successfully applied to detect residual MG in real fish samples. This work opens up an avenue to improve the reproducibility of Raman signals for flexible SERS substrates in the detection of residues under various complex conditions.

Optical Nanobiosensor Based on Surface-Enhanced Raman Spectroscopy and Catalytic Hairpin Assembly for Early-Stage Lung Cancer Detection via Blood Circular RNA

Lung cancer has become the leading cause of cancer-related deaths globally. However, early detection of lung cancer remains challenging, resulting in poor outcomes for the patients. Herein, we developed an optical biosensor integrating surface-enhanced Raman spectroscopy (SERS) with a catalyzed hairpin assembly (CHA) to detect circular RNA (circRNA) associated with tumor formation and progression (circSATB2). The signals of the Raman reporter were considerably enhanced by generating abundant SERS "hot spots" with a core-shell nanoprobe and 2D SERS substrate with calibration capabilities. This approach enabled the sensitive (limit of detection: 0.766 fM) and reliable quantitative detection of the target circRNA. Further, we used the developed biosensor to detect the circRNA in human serum samples, revealing that patients with lung cancer had higher circRNA concentrations than healthy subjects. Moreover, we characterized the unique circRNA concentration profiles of the early stages (IA and IB) and subtypes (IA1, IA2, and IA3) of lung cancer. These results demonstrate the potential of the proposed optical sensing nanoplatform as a liquid biopsy and prognostic tool for the early screening of lung cancer.

Extending the range of metal ions SERS detection using hybrid plasmonic/ZIF-8 particles

Nanosensors based on surface-enhanced Raman spectroscopy (SERS) have emerged as a class of promising optical tools for the ultrasensitive quantification of metal ions of environmental and biological interest. A central bottleneck in this field is the availability of suitable surface receptors able to convert the selective binding with these vibrationless analytes into measurable SERS signals. In this work, we tackle this issue by employing a hybrid substrate comprising a highly SERS-active plasmonic core and a ZIF-8 metal-organic framework (MOF) shell. The ZIF-8 shell firmly captures aromatic receptors close to the plasmonic structure regardless of their intrinsic affinity for the metallic surface and without altering their ability to coordinate metal ions. Furthermore, it imparts molecular sieving abilities enabling the direct use of the SERS sensing platform in complex media such as biological fluids. This was demonstrated by using different classes of chromogenic reagents (bathocuproine, a 2,6':2',2″-terpyridine derivative, and Arsenazo III) which were exploited for the SERS detection of both transition and alkaline earth metal ions (i.e., divalent copper, cobalt and calcium ions). Notably, we successfully applied this approach for the detection of Cu(II) in untreated urine samples for Wilson's disease diagnosis. Overall, we believe this class of multifunctional hybrid substrates will serve as a valuable material for expanding the applicability of SERS spectroscopy in real-life environmental and biomedical metal ions analysis.

Ag NPs@PDMS nanoripple array films as SERS substrates for rapid in situ detection of pesticide residues

The large-area fabrication of flexible and transparent surface-enhanced Raman scattering (SERS) substrates with high performance by a facile and efficient method is still challenging. Here, we demonstrated a large-scale, flexible and transparent SERS substrate composed of PDMS nanoripple array film decorated with silver nanoparticles (Ag NPs@PDMS-NR array film) prepared by a combination of plasma treatment and magnetron sputtering. The performances of SERS substrates were characterized by rhodamine 6G (R6G) using a handheld Raman spectrometer. The optimal Ag NPs@PDMS-NR array film exhibited high SERS sensitivity, with a detection limitation of R6G reaching 8.20 × 10^-8 M as well as excellent uniformity (RSD = 6.8%) and batch-to-batch reproducibility (RSD = 2.3%). In addition, the substrate showed outstanding mechanical stability and good SERS enhancement by backside illumination, thus it was suitable for in situ SERS detection on curved surfaces. The detection limit of malachite green on apple and tomato peels was 1.19 × 10^-7 and 1.16 × 10^-7 M, respectively, and quantitative analysis of pesticide residues could be realized. These results demonstrate that the Ag NPs@PDMS-NR array film has great practical potential in rapid in situ detection of pollutants.

Self-Supplied Electron Photoelectrochemical Biosensor with PTB7-Th as a Photoelectric Material and Biotin as an Efficient Quencher

In this work, a self-supplied electron photoelectrochemical (PEC) biosensor for sensitive determination of Pb²⁺ was established by utilizing donor-acceptor (D-A)-type PTB7-Th (poly{4,8-bis[5-(2-ethylhexyl) thiophen-2-yl]benzo[1,2-b,4,5-b']dithiophene-2,6-diyl-alt-3-fluoro-2-[(2-ethylhexyl)carbonyl] thieno[3,4-b]-thiophene-4,6-diyl}) as a photoelectric material coupled with biotin as an efficient signal quencher. Impressively, compared with the traditional PEC signal quenchers, biotin was first applied as a PEC signal quencher in this work and it effectively avoided a cumbersome preparation process, complex DNA sequence design, and extra reagent assistance and greatly simplified experimental steps, which could achieve an efficient PEC signal quenching toward PTB7-Th. In addition, the execution of a DNAzyme-assisted Pb²⁺ recycling amplification reaction could release the quencher biotin, leading to the recovery of the PEC signal, thereby realizing the quantitative detection of Pb²⁺. Resultantly, the submitted self-supplied electron PEC biosensor presented an extensive coverage of assay Pb²⁺ (50 fM to 500 nM) along with a low determination limit (16.7 fM), which exhibited the advantages of high selectivity and excellent stability. Importantly, this work provided a powerful alternative to traditional heavy metal-ion assessment methods and possessed the potential for application in environment, biomedicine, and food-safety fields.

Trends in Application of SERS Substrates beyond Ag and Au, and Their Role in Bioanalysis

This article compares the applications of traditional gold and silver-based SERS substrates and less conventional (Pd/Pt, Cu, Al, Si-based) SERS substrates, focusing on sensing, biosensing, and clinical analysis. In recent decades plethora of new biosensing and clinical SERS applications have fueled the search for more cost-effective, scalable, and stable substrates since traditional gold and silver-based substrates are quite expensive, prone to corrosion, contamination and non-specific binding, particularly by S-containing compounds. Following that, we briefly described our experimental experience with Si and Al-based SERS substrates and systematically analyzed the literature on SERS on substrate materials such as Pd/Pt, Cu, Al, and Si. We tabulated and discussed figures of merit such as enhancement factor (EF) and limit of detection (LOD) from analytical applications of these substrates. The results of the comparison showed that Pd/Pt substrates are not practical due to their high cost; Cu-based substrates are less stable and produce lower signal enhancement. Si and Al-based substrates showed promising results, particularly in combination with gold and silver nanostructures since they could produce comparable EFs and LODs as conventional substrates. In addition, their stability and relatively low cost make them viable alternatives for gold and silver-based substrates. Finally, this review highlighted and compared the clinical performance of non-traditional SERS substrates and traditional gold and silver SERS substrates. We discovered that if we take the average sensitivity, specificity, and accuracy of clinical SERS assays reported in the literature, those parameters, particularly accuracy (93-94%), are similar for SERS bioassays on AgNP@Al, Si-based, Au-based, and Ag-based substrates. We hope that this review will encourage research into SERS biosensing on aluminum, silicon, and some other substrates. These Al and Si based substrates may respond efficiently to the major challenges to the SERS practical application. For instance, they may be not only less expensive, e.g., Al foil, but also in some cases more selective and sometimes more reproducible, when compared to gold-only or silver-only based SERS substrates. Overall, it may result in a greater diversity of applicable SERS substrates, allowing for better optimization and selection of the SERS substrate for a specific sensing/biosensing or clinical application.

Determination of Pb2+ by Colorimetric Method Based on Catalytic Amplification of Ag Nanoparticles Supported by Covalent Organic Frameworks

In this paper, covalent organic frameworks (COFs) are prepared by solvothermal synthesis using 1,3,5-benzenetricarboxaldehyde and benzidine as ligands. Then, using COFs as a template, AgCOFs with high catalytic activity is prepared by in situ loading silver nanoparticles (AgNC) on the surface of COFs by sodium borohydride reduction method. AgCOFs are characterized by TEM, SEM, FTIR and XRD. At the same time, the catalytic ability of AgCOFs for trisodium citrate-AgNO3 nanosilver reaction is studied. The results show that AgCOFs can catalyze the reaction of trisodium citrate-AgNO3 to generate silver nanoparticles (AgNPs). The solution color of the system gradually changes from colorless to yellow, and the absorbance value increases. Based on the catalytic reaction of AgCOFs and the regulation effect of nucleic acid aptamer reaction on AgCOFs, a new "on-off-on" colorimetric analysis platform is constructed and applied to the detection of trace Pb2+ in water samples. This analytical platform is simple, sensitive and selective. Finally, the catalytic mechanism of the system is discussed to verify the feasibility of constructing a colorimetric analysis platform.

Direct Laser Writing of SERS Hollow Fibers

We report the direct laser writing (DLW) of surface-enhanced Raman scattering (SERS) structures on the inner wall of a hollow fiber. Colloidal gold-silver alloy nanoparticles (Au-Ag ANPs) are firstly coated onto the inner wall of a hollow fiber. A green laser beam is focused through the outer surface of the hollow fiber to interact with colloidal Au-Ag ANPs so that they become melted and aggregated on the surface of the inner wall with strong adhesion. Such randomly distributed plasmonic nanostructures with high density and small gaps favor the SERS detection of low-concentration molecules in liquids flowing through the hollow fiber. Such a SERS device also supplies a three-dimensional microcavity for the interaction between excitation laser and the target molecules. The DLW system consists mainly of the flexible connection between the motor shaft and the hollow fiber, the program-controlled translation of the hollow fiber along its symmetric axis and rotation about the axis, as well as the mechanical design and the computer control system. This DLW technique enables high production, high stability, high reproducibility, high precision, and a high-flexibility fabrication of the hollow fiber SERS device. The resultant microcavity SERS scheme enables the high-sensitivity detection of R6G molecules in ethanol with a concentration of 10-7 mol/L.

Recent Advances in Agglomeration Detection and Dual-Function Application of Surface-Enhanced Raman Scattering (SERS)

Surface-enhanced Raman scattering (SERS) has been widely utilized in early detection of disease biomarkers, cell imaging, and trace contamination detection, owing to its ultra-high sensitivity. However, it is also subject to certain application restrictions in virtue of its expensive detection equipment and long-term stability of SERS-active substrate. Recently, great progress has been made in SERS technology, represented by agglomeration method. Dual readout signal detection methods are combined with SERS, including electrochemical detection, fluorescence detection, etc., establishing a new fantastic viewpoint for application of SERS. In this review, we have made a comprehensive report on development of agglomeration detection and dual-function detection methods based on SERS. The synthesis methods for plasmonic materials and mainstream SERS enhancement mechanism are also summarized. Finally, the key facing challenges are discussed and prospects are addressed.

An entropy-driven signal-off DNA circuit for label-free, visual detection of small molecules with enhanced accuracy

An entropy-driven DNA circuit offers an efficient means of sensitive analyte detection with signal amplification. In this article, we rationally engineered an aptamer-based entropy-driven signal-off DNA circuit for colorimetric detection of small molecules. The proposed signal-off DNA circuit is activated by target small molecule binding to drive the collapse of G-quadruplex DNAzyme, accompanied by the colour change of the detection solution from dark blue to light blue. Entropy-driven recycling hybridization significantly magnified the input signal of the target small molecule. Such an assay enables naked-eye detection of adenosine triphosphate and oxytetracycline at concentrations as low as 0.5 μM and 1 μM respectively. Moreover, when compared with the signal-on DNA circuit, the entropy-driven signal-off DNA circuit for colorimetric detection has two advantages. Firstly, unlike in the signal-on DNA circuit, the unavoidable formation of waste complexes in the absence of a target in the signal-off DNA circuit has no influence on target detection performance as its background signal is only determined by the substrate complex. Secondly, the signal-on DNA circuit cannot distinguish false-positive signals generated by invasive catalysts (e.g., HRP, serum, Fe₃O₄), while the signal-off DNA circuit can distinguish those signals as undesired signals. Overall, the signal-off DNA circuit affords a novel strategy for sensitive and accurate detection of small molecules.

In situ fabrication of urchin-like Cu@carbon nanoneedles based aptasensor for ultrasensitive recognition of trace mercury ion

Mercury ion (Hg²⁺) is a strong toxic heavy ion that causes severe damages to the environment and readily accumulates in the food chain. However, it remains a major challenge to realize a sensitive and precise recognition of Hg²⁺ with a trace concentration for early identifying the pollution source. In this work, a novel electrochemical aptasensor was designed to achieve an ultrasensitive and quantitative detection of trace Hg²⁺, relying on an urchin-like architecture of Cu@carbon nanoneedles (Cu@CNNs) as the electroactive probe. This specific nanostructure was in-situ constructed through a controllable pyrolysis process, serving as a signal magnifier and DNA loading platform owing to its outstanding electrocatalysis and large specific surface areas. Meanwhile, an exonuclease III-assisted cycling amplification strategy was designed to efficiently amplify the signal strength of trace Hg²⁺via the consecutive release of report probes in nicking reaction. This as-prepared Hg²⁺ aptasensor exhibited an ultralow detection limit of 3.7 fM (7 × 10^-6 ppm) and a wide linear range from 10 fM to 10 μM, together with the satisfactory stability and reusability for assay in real water samples. It is highly expected that this Cu@CNNs based aptasensor will have tremendous potentials in the early warning and efficient pollution monitoring of heavy metal ions.

AAO Template-Assisted Fabrication of Ordered Ag Nanoparticles-Decorated Au Nanotubes Array for Surface-Enhanced Raman Scattering Detection

Highly sensitive and reproducible surface-enhanced Raman scattering (SERS) substrates are the main challenge for practical applications. In this work, an ordered and hierarchical Ag nanoparticles (Ag-NPs)-decorated Au nanotubes (Au-NTs) array was achieved based on a funnel-shaped pore anodic aluminum oxide (AAO) template-assisted strategy. First, funnel-pore-AAO templates were fabricated by further oxidation of conical-pore-AAO templates achieved by multistep anodization and etching. Then physical sputtering was used to assemble the Au-NTs and Ag-NPs using the as-prepared funnel-pore-AAO as sacrificial templates. SEM revealed abundant sub-10 nm neighboring gaps and sub-10 nm nanocavities at the bottom of the nanotubes because of the special shape of the AAO template, which resulted in abundant strong “hot spots” contributing to the sensitive SERS detection. The resultant hierarchical substrates manifested a SERS enhancement factor of 1.8 × 107 and reproducible response to 10−11 M rhodamine 6G and 10−8 M methyl parathion, showing potential in SERS-based rapid detection of trace pollutants in the environment.

In Situ Microfluidic SERS Chip for Ultrasensitive Hg2+ Sensing Based on I--Functionalized Silver Aggregates

Mercury(II) ions are causing serious environmental pollution and health damage. Developing a simple, rapid, and sensitive sensor for Hg²⁺ detection is of great significance. Herein, we demonstrate an I^--functionalized surface-enhanced Raman scattering (SERS) substrate for rapid and sensitive Hg²⁺ sensing on a highly integrated microfluidic platform. Based on the combination reaction between I^- and Hg²⁺, the Hg²⁺ sensing is achieved via the SERS intensity "turn-off" strategy, where HgI₂ precipitation is formed on an SERS substrate interface, dissociating the Raman reporters that coadsorbed with I^-. Owing to the strong binding constant between I^- and Hg²⁺, our I^--functionalized substrate demonstrates a very fast sensing response (∼150 s). Through reliable in situ SERS detection, a robust calibration curve between the "turn-off" signal and "lgC" is obtained in a broad concentration range of 10^-9 to 10^-13 M. Additionally, the detectable Hg²⁺ concentration can be as low as 1 fM. The good selectivity toward Hg²⁺ is also verified by testing about a dozen common metal ions in water, such as K⁺, Na⁺, Ca²⁺, Mg²⁺, and so forth. Furthermore, we apply the SERS sensor for real tap and lake water sample detection, and good recoveries of 113, 97, and 107% are obtained. With its advantages of high integration, simple preparation, fast response, high sensitivity, and reliability, the proposed I^--functionalized SERS sensor microfluidic chip can be a promising platform for real-time and on-site Hg²⁺ detection in natural water.

Fluorescence-active and peroxidase-like expanded mesoporous silica-encapsulated ultrasmall Pt nanoclusters: a novel colorimetric/fluorescent dual-mode nanosensor for sensitive detection of mercury in medicinal and edible Pueraria lobata

A novel colorimetric/fluorescent dual-mode nanosensor for Hg2+ detection was constructed using expanded mesoporous silica (EMSN)-encapsulated ultrasmall platinum nanoclusters (EMSN@Pt NCs) with improved peroxidase-like and stable fluorescent activities. The sensing technique was based on the mechanism that the peroxidase mimetic activity and fluorescence intensity of EMSN@Pt NCs can be inhibited in the presence of Hg2+. In this sensing platform, a linear range of 5-50 nM with a detection limit of 1.78±0.38 nM and quantification limit of 5.93 nM was obtained via fluorescent analysis. A linear calibration curve from 0.25 to 200 nM with a detection limit of 8.25±0.51 nM and quantification limit of 27.47 nM was achieved via colorimetric analysis. The proposed dual-mode probe possesses excellent selectivity and reliability for Hg2+ detection, which can function as an efficient nanosensor for the quantitative determination of Hg2+ in Pueraria lobata.

A capillary-based SERS sensor for ultrasensitive and selective detection of Hg2+ by amalgamation with Au@4-MBA@Ag core-shell nanoparticles

A capillary-based SERS sensor was fabricated for ultrasensitive and selective detection of Hg2+ in water. Au@Ag core-shell NPs embedded with 4-mercaptobenzoic acid (4-MBA) (Au@4-MBA@Ag) were prepared by a seed growth method and fixed on the inner wall of the glass capillary to obtain the sensor. Owing to the amalgamation between Ag and Hg, the capillary-based SERS sensor can specifically recognize the reduced Hg2+ without any recognition element, and the resulted Ag/Hg amalgam can weaken the SERS activity of Ag shell; thus, the SERS intensity of the embedded 4-MBA at 1075 cm-1 gradually decreased with the increase of Hg2+ concentration. Under the optimum condition, the fabricated sensor can sensitively determine Hg2+ in water with a limit of detection (LOD) as low as 0.03 nM. The capillary-based SERS sensor offers the advantages of simple preparation, superior stability, and high selectivity, which is promising for rapid and on-site detection of Hg2+ in water combined with a portable Raman device.

Engineering of an Upconversion Luminescence Sensing Platform Based on the Competition Effect for Mercury-Ion Monitoring in Green Tea

Accurately monitoring mercury ions (Hg²⁺) in food and agriculture-related matrixes (e.g., green tea) is of great significance to safeguard food safety. Here, we employed upconversion nanoparticles (UCNPs) and gold nanoparticles (AuNPs) to engineer a cysteine (Cys)-assisted anti-Stokes luminescence sensing platform (UCNPs-AuNPs) for precisely detecting residual Hg²⁺ in green tea through the competition effect. Initially, AuNPs could effectively quench the luminescence of UCNPs through the luminescence resonance energy transfer process, which was then interrupted by Cys-triggered AuNP aggregation via Au-S, thereby restoring UCNP luminescence. Interestingly, owing to the competition effect with AuNPs toward Cys, Hg²⁺ could weaken the luminescence restoring efficiency, achieving a Hg²⁺ concentration-dependent luminescence change. On this basis, a facile, reliable, and sensitive upconversion luminescence sensing platform for monitoring residual Hg²⁺ in green tea was successfully established. This study offers a novel insight into integrating the competition effect and anti-Stokes luminescence for food- and agriculture-related contaminant monitoring.

Quantitative Surface-Enhanced Raman Spectroscopy for Field Detections Based on Structurally Homogeneous Silver-Coated Silicon Nanocone Arrays

Practical application of surface-enhanced Raman spectroscopy (SERS) is greatly limited by the inaccurate quantitative analyses due to the measuring parameter's fluctuations induced by different operators, different Raman spectrometers, and different test sites and moments, especially during the field tests. Herein, we develop a strategy of quantitative SERS for field detection via designing structurally homogeneous and ordered Ag-coated Si nanocone arrays. Such an array is fabricated as SERS chips by depositing Ag on the template etching-induced Si nanocone array. Taking 4-aminothiophenol as the typical analyte, the influences of fluctuations in measuring parameters (such as defocusing depth and laser powers) on Raman signals are systematically studied, which significantly change SERS measurements. It has been shown that the silicon underneath the Ag coating in the chip can respond to the measuring parameters' fluctuations synchronously with and similar to the analyte adsorbed on the chip surface, and the normalization with Si Raman signals can well eliminate the big fluctuations (up to 1 or 2 orders of magnitude) in measurements, achieving highly reproducible measurements (mostly, <5% in signal fluctuations) and accurate quantitative SERS analyses. Finally, the simulated field tests demonstrate that the developed strategy enables quantitatively analyzing the highly scattered SERS measurements well with 1 order of magnitude in signal fluctuation, exhibiting good practicability. This study provides a new practical chip and reliable quantitative SERS for the field detection of real samples.

Microfluidic Transport of Hybrid Optoplasmonic Particles for Repeatable SERS Detection

For its ultrahigh sensitivity, the microfluidic system combined with surface-enhanced Raman spectroscopy (SERS) becomes one of the most interesting topics in integrated online monitoring related fields. In previous reports, the commonest surface plasmon-enhanced substrates in microfluidics consist of immobilized metal nanostructures on the channel surface to overcome the disturbance of Brownian motion. In this work, a hybrid optoplasmonic microfluidic conveyer is developed, in which the movable, highly ordered optoplasmonic particles are delivered to the detection spot for SERS detection. Here, the optoplasmonic particle is the SiO₂ microsphere with in situ photochemical reduced Ag nanoparticles on the surface. Because of the converged light at the SiO₂ microsphere surface, the SERS spectra collected at this optoplasmonic particle in the channel exhibit excellent performance, which is confirmed by the simulated electric field distribution. In addition, the experimental data also demonstrate that the quantitative analysis is achieved at 1 nM in this optoplasmonic microfluidic conveyer. Furthermore, the used optoplasmonic particle can be ejected from the microfluidic channel by modulating the velocity of injected fluid such that the new optoplasmonic particle will be delivered to the detection spot for repeatable SERS detection in the same channel. The dynamic process of optoplasmonic particle transport is investigated in this microconveyer, and the built theoretical model to predict the particle release is highly identical with the experimental data. These data point out that our hybrid optoplasmonic microfluidic conveyer has repeatable enhanced substrates with the high SERS sensitivity to overcome the cross-contamination of different target molecules in repeatable detection.

Targets and Tools: Nucleic Acids for Surface-Enhanced Raman Spectroscopy

Surface-enhanced Raman spectroscopy (SERS) merges nanotechnology with conventional Raman spectroscopy to produce an ultrasensitive and highly specific analytical tool that has been exploited as the optical signal read-out in a variety of advanced applications. In this feature article, we delineate the main features of the intertwined relationship between SERS and nucleic acids (NAs). In particular, we report representative examples of the implementation of SERS in biosensing platforms for NA detection, the integration of DNA as the biorecognition element onto plasmonic materials for SERS analysis of different classes of analytes (from metal ions to microorgniasms) and, finally, the use of structural DNA nanotechnology for the precise engineering of SERS-active nanomaterials.

Plasmonic hollow fibers with distributed inner-wall hotspots for direct SERS detection of flowing liquids

Plasmonic hollow fibers are fabricated by coating silver-/ gold-alloyed nanoparticles (Ag-Au-ANPs) onto the inner walls of hollow fibers. In this Letter, the Ag-Au-ANPs were synthesized chemically and dissolved in acetone to prepare a colloidal solution, flowed subsequently through the hollow fiber multiple times so that a thin layer of colloidal Ag-Au-ANPs was produced on the inner wall. Annealing at 400°C enabled melting/aggregation of the metallic nanoparticles and consequent formation of closely arranged plasmonic nanostructures fixed solidly on the inner wall. A surface-enhanced Raman scattering (SERS) mechanism was thus established for the liquids flowing through the hollows. The SERS measurements show an enhancement factor >10⁴ for such plasmonic hollow fibers in the direct detection of R6G/ethanol solutions. Confinement of the excitation laser energy inside the hollow space represents an additional contribution to the enhancement mechanism. This is a promising design for the direct on-site SERS detection of molecules in flowing liquids with low concentrations.

Towards a point-of-care SERS sensor for biomedical and agri-food analysis applications: a review of recent advancements

The growing demand for reliable and robust methodology in bio-chemical sensing calls for the continuous advancement of sensor technologies. Over the last two decades, surface-enhanced Raman spectroscopy (SERS) has emerged as one of the most promising analytical techniques for sensitive and trace analysis or detection in biomedical and agri-food applications. SERS overcomes the inherent sensitivity limitation associated with Raman spectroscopy, which provides vibrational "fingerprint" spectra of molecules that makes it unique and versatile among other spectroscopy techniques. This paper comprehensively reviews the recent advancements of SERS for biomedical, food and agricultural applications over the last 6 years, and we envision that, in the near future, some of these platforms have the potential to be translated as a point-of-care and rapid sensor for real-life end-user applications. The merits and limitations of various SERS sensor designs are analysed and discussed based on critical features such as sensitivity, specificity, usability, repeatability and reproducibility. We conclude by highlighting the opportunities and challenges in the field while stressing the technological gaps to be addressed in realizing commercially viable point-of-care SERS sensors for practical biomedical and agri-food technological applications.

Surface-Enhanced Raman Scattering Sensing of Transition Metal Ions in Waters

In this mini-review, we provide a coherent discussion on the sensing schemes exploited in the surface-enhanced Raman scattering (SERS) analysis of transition metal ions in waters. A critical approach was used where illustrative examples are selected to discuss key drawbacks and challenges associated with various experimental configurations and the employed enhancing substrates.

Probing blood plasma samples for the detection of diabetes using SERS aided by PCA and LDA multivariate data analyses

Fabrication of a SERS-active substrate using Langmuir–Blodgett and self-assembly techniques for the detection of diabetes from blood plasma samples.

Development of affinity between target analytes and substrates in surface enhanced Raman spectroscopy for environmental pollutant detection

Environmental pollution has long been a social concern due to the variety of pollutants and their wide distribution, persistence and being detrimental to health. It is therefore necessary to develop rapid and sensitive strategies to trace and detect these compounds. Among various detection methodologies, surface enhanced Raman spectroscopy (SERS) has become an attractive option as it enables accurate analyte identification, simple sample preparation, rapid detection and ultra-high sensitivity without any interference from water. For SERS detection, an essential yet challenging step is the effective capture of target analytes onto the surface of metal nanostructures with a high intensity of enhanced electromagnetic field. This review has systematically summarized recent advances in developing affinity between targets and the surface of SERS substrates via direct adsorption, hydrophobic functional groups, boronate affinity, metal organic frameworks (MOFs), DNA aptamers and molecularly imprinted polymers (MIPs). At the end of this review, technical limitations and outlook have been provided, with suggestions on optimizing SERS techniques for real-world applications in environmental pollutant detection.

Raman reporter-assisted Au nanorod arrays SERS nanoprobe for ultrasensitive detection of mercuric ion (Hg2+) with superior anti-interference performances

Ultra sensitive detection of mercuric ion (Hg²⁺) with superior anti-interference capability from natural water is of great significance for food safety, environmental protection, and human health. We herein develop Au ordered nanorod arrays (Au NRAs) as surface-enhanced Raman scattering (SERS) substrates to construct SERS-active and signal-reproducible sensing platforms modified with 4-mercaptophenylboronic acid (4-MBA) as multifunctional SERS reporters. The aqueous Hg²⁺ can be efficiently trapped by 4-MBA through electrophilic substitution reactions and precisely appraise its concentration based on the collective spectral changes of reporters including peak disappearance, emergence, and Raman shift. Based on this, the optical nanoprobe shows an ultrahigh detection sensitivity of 0.1 nM for Hg²⁺, which is two orders of magnitude lower than the U.S.A. environmental protection agency (EPA)-required maximum level of drinkable water. It also offers both an exceptional Hg²⁺ discrimination against other metal ions as well as organic ligands and perfect feasibilities of detecting solutions with ultra-wide pH ranges from 1.0-14.0 at varying temperatures. Moreover, the nanoprobe demonstrates an ability to identify different chemical forms of mercury and has a high repeatability, accuracy and reliability to meet the practical detection requirements in natural environments.

Application of molecular SERS nanosensors: where we stand and where we are headed towards?

Molecular specific and highly sensitive detection is the driving force of the surface-enhanced Raman spectroscopy (SERS) community. The technique opens the window to the undisturbed monitoring of cellular processes in situ or to the quantification of small molecular species that do not deliver Raman signals. The smart design of molecular SERS nanosensors makes it possible to indirectly but specifically detect, e.g. reactive oxygen species, carbon monoxide or potentially toxic metal ions. Detection schemes evolved over the years from simple metallic colloidal nanoparticles functionalized with sensing molecules that show uncontrolled aggregation to complex nanostructures with magnetic properties making the analysis of complex environmental samples possible. The present article gives the readership an overview of the present research advancements in the field of molecular SERS sensors, highlighting future trends.

Au Sputtered Paper Chromatography Tandem Raman Platform for Sensitive Detection of Heavy Metal Ions

Surface-enhanced Raman scattering (SERS) is a powerful technique for sensitive detection, but it normally has difficulty in multicomponent detection in a complex system, especially for simultaneous analysis of mixture of heavy metal ions. In this work, a simple paper chromatography tandem SERS (PC-SERS) separation/detection platform is proposed by ion-sputtering gold on a filter paper. Based on SEM results, the great electromagnetic field inside nanogaps of Au nanoislands on the paper surface is evaluated with FDTD simulation. It is found that the PC-SERS platform has good uniformity (RSD = 10.12%) and long-time stability. The as-prepared PC-SERS platform was applied to efficiently separate and detect a mixture of pesticides (MG, MB, and CV) in pond water without any pretreatment process, and the limits of detection (LODs) were down to 10 nM. As a crucial application for food safety, several heavy metal ions such as Cd²⁺, Cu²⁺, and Ni²⁺ in grinded rice were successfully detected by the PC-SERS method taking advantage of the sandwich structure based on 4-mercaptobenzoic acid (4-MBA) molecules, which were modified onto sputtering the Au filter paper and gold nanoparticles (Au NPs) to link metal ions and acted as Raman signal molecules. All the LODs for metal ions were down to 1 μM. Due to the easiness of fabrication, good reproducibility, and simple pretreatment step, the PC-SERS platform holds promise in multicomponent detection in a real sample.

Biomimetic preparation of core-shell structured surface-enhanced Raman scattering substrate with antifouling ability, good stability, and reliable quantitative capability

The fouling and stability are two most critical limiting factors for practical applications of surface-enhanced Raman scattering (SERS)-based microfluidic electrophoresis device. Herein, we present a novel biomimetic nanoengineering strategy to achieve a SERS substrate featuring antifouling ability, good stability, and reliable quantitative capability. Typically, by employing tea polyphenol as the reducing agent, the substrate made of silver core-gold shell nanostructures in situ grown on silicon wafer surface is fabricated. The core-shell nanostructures are further embedded with internal standard molecules. Remarkably, the fabricated substrate preserves distinct SERS effects, adaptable reproducibility, and reliable quantitative ability even if the substrate is incubated with 15% H₂ O₂ , 13% HNO₃ , or 10⁸  CFU/mL bacteria, or suffered from 12-day continuous vibration at 250 rpm/min in PBS buffer. As a proof-of-concept application, the DNA-functionalized substrate is capable of precise quantification of Hg²⁺ with a limit of detection down to ca. 1 pM even in sewage water.

Wavelength distinguishable signal quenching and enhancing toward photoactive material 3,4,9,10-perylenetetracarboxylic dianhydride for simultaneous assay of dual metal ions

Photoelectrochemical (PEC) assay with low background, simple instrumentation and high sensitivity has deemed as one of the most potential strategies to simultaneous multi-component detection. How to distinguish photocurrent changes caused by various targets on a single sensing platform thus becomes the key issue to be resolved. Herein, we innovatively proposed a multiplex PEC biosensor based on wavelength distinguishable signal quenching and enhancing toward photoactive material 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) for simultaneous assay of dual metal ions. Briefly, S1 and S2 ssDNA containing sensitizer methylene blue and quencher ferrocene (termed as MB-S1 and Fc-S2), respectively, were first generated through target Pb²⁺ and Mg²⁺-induced DNAzyme-assisted target recycling, which thereafter were modified on PTCDA sensing platform specifically via host-guest recognition with β-cyclodextrin (β-CD). Interestingly, the sensitizer MB could enhance photocurrent of PTCDA under the excitation wavelength of 623 nm and 590 nm, respectively, while the quencher Fc just quencher the photocurrent of PTCDA under the excitation wavelength of 590 nm, thereby achieving wavelength distinguishable signal quenching and enhancing toward photoactive material PTCDA for simultaneous assay of dual metal ions. As a result, the conceived biosensor for Mg²⁺ and Pb²⁺ detection realized high sensitivity with detection limit of 0.3 pM and 0.3 nM, respectively. The proposed strategy not only for the first time achieved the discrimination of varied PEC signal caused by two targets with usage of sole photoelectric material, but also realized the simultaneous multiplex assay on a single sensing platform, providing a new way for constructing effective and sensitive PEC biosensor for multi-component detection.

Hierarchically spacing DNA probes on bio-based nanocrystal for spatial detection requirements

Sterically spacing and locating functional matters at the nanoscale exert critical effects on their application, especially for the fluorescence probes whose aggregation causes emission quenching. Here we achieved a hierarchical spacing strategy of DNA fluorescence probes for ion detection via locating them separately on rod-like cellulose nanocrystals (CNCs) and further isolating CNCs by pre-grafting long molecular chains. Controlling chemical structure of CNC and location degree could adjust the inter-space of DNA probes (with a molecular length of ca. 3.6 nm) in a range of 3.5-6.5 nm with a gradient about 0.2 nm. A length up to micrometer scale of the CNC nanorods was necessary to provide DNA probes with well-separated grafting locations and enough freedom, which brought a vast linear detection range from 10 nmol/L to 5 μmol/L of Hg²⁺ concentration. The abundant reactive sites on CNC allowed a grafting pre-location of poly (tert-butyl acrylate) (PtBA) to promote the isolation of DNA probes. Controlled radical polymerization was employed to adjust the length of PtBA molecular chains, which increased the linear sensitivity coefficient of Hg²⁺ detection by ca. 2.5 times. This hierarchical nanoscale spacing concept based on chemical design can hopefully conduce to the development of biosensor and medical diagnosis. A hierarchical spacing strategy was applied to separate DNA fluorescent probes on CNCs and detect ion concentration linearly. The first-level spacing was to locate probes uniformly on CNCs, obtaining a wide linear range; and the second-level spacing was to isolate CNCs with polymer, obtaining an increased linear coefficient.

Au@gap@AuAg Nanorod Side-by-Side Assemblies for Ultrasensitive SERS Detection of Mercury and its Transformation

As one of the most toxic heavy metal elements, mercury ion (Hg²⁺ ) and its methylated product, methylmercury (MeHg) can pose a threat to human health and the environment. Herein, a novel Raman biosensor with cascade sensitivity is developed for Hg²⁺ detection through Au@gap@AuAg nanorod side-by-side assemblies. Due to the strong electromagnetic coupling from the assemblies and core-shell structure, the Raman sensor possesses high sensitivity with the limit of detection (LOD) of 0.001 ng mL^-1 , which is about one order lower than traditional atomic fluorescence spectrometer (AFS) methods. Moreover, the fabricated biosensor is used to measure residual mercury levels in tissues and eggs of hens fed high-mercury diets, and the results show total mercury in collected egg yolks is 20 times higher than whites. Furthermore, the form of mercury in the eggs is also analyzed by high-performance liquid chromatography coupled with AFS, and, unexpectedly, the methylated product MeHg tends to only be found in egg whites. These interesting differences may indicate a new research direction for the toxicity of mercury in living organisms, and the developed ultrasensitive Surface Enhanced Raman Scattering (SERS) method could pave a broad way for the application of biosensors in Hg detection.

A "turn-off" SERS aptasensor based DNAzyme-gold nanorod for ultrasensitive lead ion detection

It is great significance to precisely monitor lead (II) ions (Pb²⁺) for environment protection and human health monitoring. We designed a sensitive detection strategy for sensitive and selective determination of Pb²⁺, based on a Pb²⁺-specific DNAzyme as the catalytic unit, Cy3-labeled DNA modified gold nanorods (AuNRs) as SERS reporter. Firstly, AuNRs surface were employed as a platform for the immobilization of thiolated probe DNA, and then hybridized with DNAzyme catalytic beacons. By taking advantage of DNAzyme digest, a molecular beacon, causes a "turn-off" SERS signal by disrupting the labeled probes. Under the optical conditions, the DNAzyme-AuNRs sensor system exhibited high sensitivity, acceptable stability and reproducibility with a wide linear range from 0.5 to 100 nM (R² = 0.9973), and an ultra-low detection limit of 0.01 nM. The proposed strategy has additional advantages of being less time-consuming, low-cost and remote query, and avoids the interference of some metals such as Fe³⁺, Cd²⁺, Ba²⁺, Cu²⁺, Zn²⁺. The SERS biosensor system has been successfully applied for detecting Pb²⁺ in real samples with a satisfactory result. The result indicated that the proposed sensing strategy not only enriches SERS platform of monitoring Pb²⁺ but also exhibits potential for the point-of-care diagnostic application of the clinical screening in complicated biological samples.