Electrochemically Created Highly Surface Roughened Ag Nanoplate Arrays for SERS Biosensing Applications

An Ag-nanoplate decorated cavity-nanorod array SERS substrate for trace detection of PCB-77

We report the fabrication of a substrate with cavity-nanorods and decorated with Ag-nanoplates (C-NR@Ag). The cavities on the substrate are formed by metal assistant chemical etching, and the Ag-nanoplates in the cavities by galvanic cell deposition enhance the SERS performance effectively. Analytes in solution are adsorbed on Ag-nanoplates and located in hot spots, which enhance the SERS performance effectively. The enhancement factor of the Ag-nanoplates decorated on nanorod cavities is calculated to be 3.6 × 106, which is about 3 fold higher than that on the nanorods. The C-NR@Ag substrate is able to detect polychlorinated biphenyls (PCBs) with the lower limit of detection at 1.0 × 10-12 M. Additionally, due to the semi-volatile nature of PCB-77, the lower limit of detection of the C-NR@Ag substrate for PCB-77 was 1.0 × 10-11 M by the non-contact collection method. These results present a novel approach towards enhancing SERS performance and facilitating the rapid detection of PCB-77.

Sensors and Devices Guided by Artificial Intelligence for Personalized Pain Medicine

Personalized pain medicine aims to tailor pain treatment strategies for the specific needs and characteristics of an individual patient, holding the potential for improving treatment outcomes, reducing side effects, and enhancing patient satisfaction. Despite existing pain markers and treatments, challenges remain in understanding, detecting, and treating complex pain conditions. Here, we review recent engineering efforts in developing various sensors and devices for addressing challenges in the personalized treatment of pain. We summarize the basics of pain pathology and introduce various sensors and devices for pain monitoring, assessment, and relief. We also discuss advancements taking advantage of rapidly developing medical artificial intelligence (AI), such as AI-based analgesia devices, wearable sensors, and healthcare systems. We believe that these innovative technologies may lead to more precise and responsive personalized medicine, greatly improved patient quality of life, increased efficiency of medical systems, and reducing the incidence of addiction and substance use disorders.

Label-free detection and discrimination of respiratory pathogens based on electrochemical synthesis of biomaterials-mediated plasmonic composites and machine learning analysis

Seasonal outbreaks of respiratory viral infections remain a global concern, with increasing morbidity and mortality rates recorded annually. Timely and false responses contribute to the widespread of respiratory pathogenic diseases owing to similar symptoms at an early stage and subclinical infection. The prevention of emerging novel viruses and variants is also a big challenge. Reliable point-of-care diagnostic assays for early infection diagnosis play a critical role in the response to threats of epidemics or pandemics. We developed a facile method for specifically identifying different viruses based on surface-enhanced Raman spectroscopy (SERS) with pathogen-mediated composite materials on Au nanodimple electrodes and machine learning (ML) analyses. Virus particles were trapped in three-dimensional plasmonic concave spaces of the electrode via electrokinetic preconcentration, and Au films were simultaneously electrodeposited, leading to the acquisition of intense and in-situ SERS signals from the Au-virus composites for ultrasensitive SERS detection. The method was useful for rapid detection analysis (<15 min), and the ML analysis for specific identification of eight virus species, including human influenza A viruses (i.e., H1N1 and H3N2 strains), human rhinovirus, and human coronavirus, was conducted. The highly accurate classification was achieved using the principal component analysis-support vector machine (98.9%) and convolutional neural network (93.5%) models. This ML-associated SERS technique demonstrated high feasibility for direct multiplex detection of different virus species for on-site applications.

High-density Ag nanosheets for selective electrochemical CO2 reduction to CO

To increase the specific surface area, high-density (i.e. number per unit area) Ag nanosheets (ANS) with large electrochemically active surface area and rich edge active sites over Ag plates were synthesized via a facile electrodeposition approach in a double electrode system at a constant current of -1 mA for 1800 s. By adjusting the concentration of H₃BO₃ (0.5 M, 0.1 M and 0.05 M), which is used to control the growth direction of ANS, ANS-20, -50, -350 were obtained with varying thickness of 20 nm, 50 nm, and 350 nm, respectively. Notably, ANS-20 showed a remarkable current density of -6.48 mA cm^-2 at -0.9 V versus the reversible hydrogen electrode (RHE), which is almost 1.6 and 2.4 times as high as those of ANS-50 and -350, respectively. Furthermore, ANS-20 exhibits the best CO selectivity of 91.2% at -0.8 V versus RHE, while the other two give 84.6% and 77.9% at the same potential. The excellent performance of ANS-20 is attributed to its rich edge active sites and large electrochemically active surface area (ECSA).

Urchin-like ZnO-nanorod arrays templated growth of ordered hierarchical Ag/ZnO hybrid arrays for surface-enhanced Raman scattering

Here, a universal strategy for the controllable synthesis of three dimensional (3D) hierarchical Ag/ZnO hybrid arrays based on the urchin-like ZnO-nanorod array template is presented. The urchin-like ZnO-nanorod arrays are first achieved by electrodepositing a high density of ZnO-nanorods onto the surface of highly hexagonally arranged arrays of polystyrene (PS) microspheres, and then Ag-nanoparticles (Ag-NPs) are assembled onto the surface of each ZnO-nanorod via photochemical reaction, ion sputtering, galvanic cell reaction deposition and electrochemical deposition, forming the ordered hierarchical Ag/ZnO hybrid arrays. The urchin-like Ag/ZnO hybrid arrays with well-ordered hierarchical morphology and high density 'hot spots' located in the sub-10 nm gaps between neighboring Ag-NPs on both the same ZnO-nanorod and neighboring ZnO-nanorods can be directly utilized as hybrid surface-enhanced Raman scattering (SERS) substrates with high SERS activity. This work provides a strategy for the rational assembly of well-ordered hierarchical noble metal/semiconductor hybrid arrays, which may open up many opportunities in areas such as catalysis, SERS, and biosensing.

Optothermal microbubble assisted manufacturing of nanogap-rich structures for active chemical sensing

Guiding analytes to the sensing area is an indispensable step in a sensing system. Most of the sensing systems apply a passive sensing method, which waits for the analytes to diffuse towards the sensor. However, passive sensing methods limit the detection of analytes to a picomolar range on micro/nanosensors for a practical time scale. Therefore, active sensing methods need to be used to improve the detection limit in which the analytes are forced to concentrate on the sensors. In this article, we have demonstrated the manufacturing of nanogap-rich structures for active chemical sensing. Nanogap-rich structures are manufactured from metallic nanoparticles through an optothermally generated microbubble (OGMB) which is a laser-induced micron-sized bubble. The OGMB induces a strong convective flow that helps to deposit metallic nanoparticles to form nanogap-rich structures on a solid surface. In addition, the OGMB is used to guide and concentrate analytes towards the nanogap-rich structures for the active sensing of analytes. An active sensing method can improve the detection limit of chemical substances by an order of magnitude compared to a passive sensing method. The microbubble assisted manufacturing of nanogap-rich structures together with an active analyte sensing method paves a new way for advanced chemical and bio-sensing applications.

Fabrication of Ag-nanosheets-built micro/nanostructured arrays via in situ conversion on Cu2O-coated Si nanocone platform and their highly structurally-enhanced SERS effect

A controllable and flexible route is presented for the fabrication of Ag-nanosheets-built micro/nanostructural ordered arrays via in situ conversion on the Cu₂O-coated silicon nanocone (SNC) platform in the AgNO₃-contained solution. The platform is pre-prepared by the reactive ion etching of the organic colloidal monolayer-covered silicon wafer, Cu sputtering deposition and in situ oxidation. The obtained Ag micro/nanostructured array consists of nearly spherical and micro-sized particles, which are hexagonally arranged on the substrate. The spherical particles are built of the vertically standing and cross-linked nanosheets with about 30 nm in thickness. This Ag-nanosheets-built array shows high number density of the edges and nanogaps as well as the robust and homogeneous structure. Its formation is attributed to the in situ conversion reaction on the Cu₂O-coated SNC platform and the preferentially-oriented connection of Ag nanoparticles. Such Ag array has shown significantly higher surface enhanced Raman scattering (SERS) activity than the Ag nanoparticles' film-covered SNC array, with the enhancement factor up to 10⁷ and the detection limitation down to ∼1 ppt level to the test molecules 4-aminothiophenol, as well as the good reproducibility in measurements. This study not only presents a controllable and flexible fabrication route to the plasmonic micro/nanostructured arrays but also provides the highly efficient and the practical chips for the SERS-based devices.

Recent Advances in 2D Inorganic Nanomaterials for SERS Sensing

Surface-enhanced Raman spectroscopy is a powerful and sensitive analytical tool that has found application in chemical and biomolecule analysis and environmental monitoring. Since its discovery in the early 1970s, a variety of materials ranging from noble metals to nanostructured materials have been employed as surface enhanced Raman scattering (SERS) substrates. In recent years, 2D inorganic materials have found wide use in the development of SERS-based chemical sensors owing to their unique thickness dependent physico-chemical properties with enhanced chemical-based charge-transfer processes. Here, recent advances in the application of various 2D inorganic nanomaterials, including graphene, boron nitride, semiconducting metal oxides, and transition metal chalcogenides, in chemical detection via SERS are presented. The background of the SERS concept, including its basic theory and sensing mechanism, along with the salient features of different nanomaterials used as substrates in SERS, extending from monometallic nanoparticles to nanometal oxides, is comprehensively discussed. The importance of 2D inorganic nanomaterials in SERS enhancement, along with their application toward chemical detection, is explained in detail with suitable examples and illustrations. In conclusion, some guidelines are presented for the development of this promising field in the future.

Simultaneous detection of different probe molecules using silver nanowires as SERS substrates

Metallic silver nanowires with high yield were synthesized using a modified seed mediated approach at room temperature. Ribbon-like nanostructures were obtained when the concentration of NaOH was lower and further increase of NaOH transformed it into long nanowires. These nanowires possess high aspect ratio, with length and diameter ~6.5 μm and 17 nm respectively. The surface enhanced Raman scattering activity of these nanowires was tested with three different probe molecules viz., crystal violet, malachite green and nile blue chloride using visible (514.4 nm) and near-infrared (784.8 nm) excitation lines. The minimum detection limits for crystal violet and nile blue chloride molecules were found to be down to 10^-7 M with good linear responses, as evidenced by values of correlation coefficients, indicating their potential for a variety of applications such as sensing. Principal component analysis was performed with the surface enhanced Raman spectra in order to discriminate the dye molecules and their mixture, simultaneously. The first two principal components, which provided 69.80 and 27.93% of the total data variance, could be conveniently represented as a two dimensional PCA score plot. The score plot showed clear clustering of probe molecules and their mixture. The relative contribution of wavenumbers to each of the two principal components was identified by plotting the PCA loading matrix. These results further promote possibilities of quantification of multiplexed SERS detection and analysis.

A green and general strategy for the synthesis of hollow Ag/CdS nanocomposites for superior SERS performance

In this study, we developed a convenient, environmentally friendly approach for the fabrication of hollow Ag/CdS composites, which presented superior SERS performance.

Self-Assembly of Au@Ag Nanoparticles on Mussel Shell To Form Large-Scale 3D Supercrystals as Natural SERS Substrates for the Detection of Pathogenic Bacteria

Herein, we developed a natural surface-enhanced Raman scattering (SERS) substrate based on size-tunable Au@Ag nanoparticle-coated mussel shell to form large-scale three-dimensional (3D) supercrystals (up to 10 cm2) that exhibit surface-laminated structures and crossed nanoplates and nanochannels. The high content of CaCO3 in the mussel shell results in superior hydrophobicity for analyte enrichment, and the crossed nanoplates and nanochannels provided rich SERS hot spots, which together lead to high sensitivity. Finite-difference time-domain simulations showed that nanoparticles in the channels exhibit apparently a higher electromagnetic field enhancement than nanoparticles on the platelets. Thus, under optimized conditions (using Au@AgNPs with 5 nm shell thickness), highly sensitive SERS detection with a detection limit as low as 10-9 M for rhodamine 6G was obtained. Moreover, the maximum electromagnetic field enhancement of different types of 3D supercrystals shows no apparent difference, and Au@AgNPs were uniformly distributed such that reproducible SERS measurements with a 6.5% variation (613 cm-1 peak) over 20 spectra were achieved. More importantly, the as-prepared SERS substrates can be utilized for the fast discrimination of Escherichia coli, Staphylococcus aureus, and Pseudomonas aeruginosa by discriminant analysis. This novel Au@Ag self-assembled mussel shell template holds considerable promise as low-cost, durable, sensitive, and reproducible substrates for future SERS-based biosensors.

Metal-Organic Framework-Based Sensors for Environmental Contaminant Sensing

Increasing demand for timely and accurate environmental pollution monitoring and control requires new sensing techniques with outstanding performance, i.e., high sensitivity, high selectivity, and reliability. Metal-organic frameworks (MOFs), also known as porous coordination polymers, are a fascinating class of highly ordered crystalline coordination polymers formed by the coordination of metal ions/clusters and organic bridging linkers/ligands. Owing to their unique structures and properties, i.e., high surface area, tailorable pore size, high density of active sites, and high catalytic activity, various MOF-based sensing platforms have been reported for environmental contaminant detection including anions, heavy metal ions, organic compounds, and gases. In this review, recent progress in MOF-based environmental sensors is introduced with a focus on optical, electrochemical, and field-effect transistor sensors. The sensors have shown unique and promising performance in water and gas contaminant sensing. Moreover, by incorporation with other functional materials, MOF-based composites can greatly improve the sensor performance. The current limitations and future directions of MOF-based sensors are also discussed.

Nonlithographic Fabrication of Plastic-Based Nanofibers Integrated Microfluidic Biochip for Sensitive Detection of Infectious Biomarker

We report fabrication of a fully integrated plastic based microfluidic biochip for biosensing application. The microfluidic channels were fabricated by tune transfer method and integrated with the prefunctionalized sensing platform. This approach to assembling microchannels into prefunctionalized sensing substrate facilitates controlled functionalization and prevents damages on the functionalized surface. The sensing platform comprised a three-electrode system, in which the sensing electrode was integrated with antibody immobilized carbon nanotubes-zinc oxide (C-ZnO) nanofibers. Electrospinning technique was used to synthesize C-ZnO nanofibers and the surface of the nanofibers was covalently conjugated with histidine rich protein II antibodies (AntiHRP II) toward detection of infectious malarial specific antigen, namely histidine-rich protein II (HRP II). The analytical performance of the fabricated biochip was evaluated by differential pulse voltammetry method. The device exhibited a high sensitivity of 1.19 mA/((g mL^-1)/cm²) over a wide detection range (10 fg/mL to 100 μg/mL) with a low detection limit of 7.5 fg/mL toward HRP II detection. This fully integrated biochip offers a promising cost-effective approach for detection of several other infectious disease biomarkers.

Investigation of simultaneously existed Raman scattering enhancement and inhibiting fluorescence using surface modified gold nanostars as SERS probes

One of the main challenges for highly sensitive surface-enhanced Raman scattering (SERS) detection is the noise interference of fluorescence signals arising from the analyte molecules. Here we used three types of gold nanostars (GNSs) SERS probes treated by different surface modification methods to reveal the simultaneously existed Raman scattering enhancement and inhibiting fluorescence behaviors during the SERS detection process. As the distance between the metal nanostructures and the analyte molecules can be well controlled by these three surface modification methods, we demonstrated that the fluorescence signals can be either quenched or enhanced during the detection. We found that fluorescence quenching will occur when analyte molecules are closely contacted to the surface of GNSs, leading to a ~100 fold enhancement of the SERS sensitivity. An optimized Raman signal detection limit, as low as the level of 10-11 M, were achieved when Rhodamine 6 G were used as the analyte. The presented fluorescence-free GNSs SERS substrates with plentiful hot spots and controllable surface plasmon resonance wavelengths, fabricated using a cost-effective self-assembling method, can be very competitive candidates for high-sensitive SERS applications.

Asymmetric bead aggregation for microfluidic immunodetection

We report an asymmetric immunoaggregation assay for rapid, label-free, and sub-picomolar protein detection. Asymmetric immunoaggregated beads (AIBs) are formed when binding occurs between 1 μm magnetic (MG) and 2.8 μm polystyrene (PS) beads coated with specific antibodies for a target antigen. Detection of such aggregation is achieved by optical monitoring of AIBs in a flow under an external magnetic field. AIBs are attracted to the upper surface of the microchannel by a magnetic field and made to slide along the surface by a flow drag force. This sliding behavior is in contrast with other particles such as MG and PS beads; while attracted MG beads hardly slide due to their small size, PS beads quickly move with the flow due to the lack of magnetism. Sliding AIBs are optically monitored in a designated sensing area in the microchannel. A custom-built program code is used for counting the AIBs and further analysis of parameters such as velocity and number distributions that are correlated with target concentrations. The detection range from 54 pg mL^-1 to 54 ng mL^-1 is demonstrated for the influenza type A H1N1 nucleoprotein (NP). This immunosensing system is simple, highly sensitive, and capable of quantitative detection of antigens in a single test without fluorescent or enzyme labeling, hence is useful for the rapid detection of biomarkers in clinical and biomedical applications.

Raman scattering enhanced within the plasmonic gap between an isolated Ag triangular nanoplate and Ag film

Enhanced electromagnetic field in the tiny gaps between metallic nanostructures holds great promise in optical applications. Herein, we report novel out-of-plane nanogaps composed of micrometer-sized Ag triangular nanoplates (AgTN) on Ag films. Notably, the new coupled plasmonic structure can dramatically enhance the surface-enhanced Raman scattering (SERS) by visible laser excitation, although the micrometer-sized AgTN has localized plasmon resonance at infrared wavelength. This enhancement is derived from the gap plasmon polariton between the AgTN and Ag film, which is excited via the antenna effect of the corner and edge of the AgTN. Systematic SERS studies indicated that the plasmon enhancement was on the order of corner > edge > face. These results were further verified by theoretical simulations. Our device paves the way for rational design of sensitive SERS substrates by judiciously choosing appropriate nanoparticles and optimizing the gap distance.

Electrodeposition of High Density Silver Nanosheets with Controllable Morphologies Served as Effective and Reproducible SERS Substrates

Silver nanosheets with a nanogap smaller than 10 nm and high reproducibility were constructed through simple and environmentally friendly electrodeposition method on copper plate. The sizes of the nanogaps can be varied from around 7 to 150 nm by adjusting the deposition time and current density. The nanosheets with different nanogaps exhibited varied surface-enhanced Raman scattering (SERS) properties due to electromagnetic mechanism (EM). The optimized high density silver nanosheets with a nanogap smaller than 10 nm showed effective SERS ability with an enhanced factor as high as 2.0 × 10(5). Furthermore, the formation mechanism of the nanosheets during the electrodeposition process has been investigated by discussing the influence of boric acid and current density. This method has proved to be applicable on different metal substrates, which exhibits the potential to be widely used in different fields.

A multiplexed immunoaggregation biomarker assay using a two-stage micro resistive pulse sensor

We present an immunoaggregation assay chip for multiplexed biomarkers detection. This chip is based on immunoaggregation of antibody functionalized microparticles (Ab-MPs) to quantify concentrations of multiple biomarkers simultaneously. A mixture of multiple types of Ab-MPs probes with different sizes and magnetic properties, which were functionalized by different antibodies, was used for the multiplexed assay. The interactions between biomarkers and their specific Ab-MPs probes caused the immunoaggregation of Ab-MPs. A two-stage micro resistive pulse sensor was used to differentiate and count the Ab-MP aggregates triggered by different biomarkers via size and magnetic property for multiplexed detection. The volume fraction of each type of Ab-MP aggregates indicates the concentration of the corresponding target biomarker. In our study, we demonstrated multiplexed detection of two model biomarkers (human ferritin and mouse anti-rabbit IgG) in 10% fetal bovine serum, using anti-ferritin Ab and anti-mouse IgG Ab functionalized MPs. We found that the volume fraction of Ab-MP aggregates increased with the increased biomarker concentrations. The detection ranges from 5.2 ng/ml to 208 ng/ml and 3.1 ng/ml to 5.12 × 10(4 )ng/ml were achieved for human ferritin and mouse anti-rabbit IgG. This bioassay chip is able to quantitatively detect multiple biomarkers in a single test without fluorescence or enzymatic labeling process and hence is promising to serve as a useful tool for rapid detection of multiple biomarkers in biomedical research and clinical applications.

A monolayer of hierarchical silver hemi-mesoparticles with tunable surface topographies for highly sensitive surface-enhanced Raman spectroscopy

We proposed a facile green synthesis system to synthesize large-scale Ag hemi-mesoparticles monolayer on Cu foil. Ag hemi-mesoparticles have different surface morphologies on their surfaces, including ridge-like, meatball-like, and fluffy-like shapes. In the reaction, silver nitrate was reduced by copper at room temperature in dimethyl sulfoxide via the galvanic displacement reaction. The different surface morphologies of the Ag hemi-mesoparticles were adjusted by changing the reaction time, and the hemi-mesoparticle surface formed fluffy-spherical nanoprotrusions at longer reaction time. At the same time, we explored the growth mechanism of silver hemi-mesoparticles with different surface morphologies. With 4-mercaptobenzoic acid as Raman probe molecules, the fluffy-like silver hemi-mesoparticles monolayer with the best activity of surface enhanced Raman scattering (SERS), the enhancement factor is up to 7.33 × 10(7) and the detection limit can reach 10(-10)M. SERS measurements demonstrate that these Ag hemi-mesoparticles can serve as sensitive SERS substrates. At the same time, using finite element method, the distribution of the localized electromagnetic field near the particle surface was simulated to verify the enhanced mechanism. This study helps us to understand the relationship between morphology Ag hemi-mesoparicles and the properties of SERS.

Monolayer graphene on nanostructured Ag for enhancement of surface-enhanced Raman scattering stable platform

We have reported that the Ag nanostructure-based substrate is particularly suitable for surface-enhanced Raman scattering when it is coated with monolayer graphene, an optically transparent and chemistry-inertness material in the visible range. Ag bowtie nanoantenna arrays and Ag nanogrids were fabricated using plasma-assisted nanosphere lithography. Our measurements show that atmospheric sulfur containing compounds are powerless to break in the monolayer graphene to vulcanize the surfaces of the Ag bowtie nanoantenna arrays and Ag nanogrids by various means, including scanning electron microscopy (SEM) and x-ray photoelectron spectroscopy (XPS). Furthermore, the Ag nanostructure substrate coated with the monolayer graphene film shows a larger enhancement of Raman activity and the electromagnetic field than the uncoated substrate. Compared with those of bare Ag nanostructures, the averaged EFs of graphene-film-coated Ag nanostructures were estimated to be about 21 and 5 for Ag bowtie nanoantenna arrays and nanogrids after one month later in air, respectively. These observations are further supported by theoretical calculations.