Electrochemical Surface-Enhanced Raman Spectroscopy as a Platform for Bacterial Detection and Identification

Surface-Enhanced Raman Spectroscopy and Electrochemistry: The Ultimate Chemical Sensing and Manipulation Combination

One of the lessons we learned from the COVID-19 pandemic is that the need for ultrasensitive detection systems is now more critical than ever. While sensors' sensitivity, portability, selectivity, and low cost are crucial, new ways to couple synergistic methods enable the highest performance levels. This review article critically discusses the synergetic combinations of optical and electrochemical methods. We also discuss three key application fields-energy, biomedicine, and environment. Finally, we selected the most promising approaches and examples, the open challenges in sensing, and ways to overcome them. We expect this work to set a clear reference for developing and understanding strategies, pros and cons of different combinations of electrochemical and optical sensors integrated into a single device.

Advances in electrochemical-optical dual-mode biosensors for detection of environmental pathogens

Electrochemical techniques are commonly used to analyze and screen various environmental pathogens. When used in conjunction with other optical recognition methods, it can extend the sensing range, lower the detection limit, and offer mutual validation. Nowadays, electrochemical-optical dual-mode biosensors have ensured the accuracy of test results by integrating two signals into one, indicating their potential use in primary food safety quantitative assays and screening tests. Particularly, visible optical signals from electrochemical/colorimetric dual-mode biosensors could meet the demand for real-time screening of microbial pathogens. While electrochemical-optical dual-mode probes have been receiving increasing attention, there is limited emphasis on the design approaches for sensors intended for microbial pathogens. Here, we review the recent progress in the merging of optical and electrochemical techniques, including fluorescence, colorimetry, surface plasmon resonance (SPR), and surface enhanced Raman spectroscopy (SERS). This study particularly emphasizes the reporting of various sensing performances, including sensing principles, types, cutting-edge design approaches, and applications. Finally, some concerns and upcoming advancements in dual-mode probes are briefly outlined.

Modified exfoliated graphene functionalized with carboxylic acid-group and thionine on a screen-printed carbon electrode as a platform for an electrochemical enzyme immunosensor

An enzyme immunoassay was developed based on the coulometric measurement of immunoglobulin M (IgM) against Hantaan viruses (HTNV) by using virus-like particles (VLPs) as recognition molecules. The surface functionalization of screen-printed carbon electrodes (SPCEs) was achieved through paste-exfoliated graphene that was modified with a COOH group and a thionine mediator through supramolecular-covalent scaffolds, on SPCEs by using the binder contained in the ink. After the covalent immobilization of the antibody, the sensor was used for the sandwich enzyme immunoassay of IgM against HTNV. By using HTNV VLPs as the second recognization molecules, the resulting sensor efficiently monitored the reaction of IgM against HTNV and anti-IgM antibody with high specificity. By attaching HTNV nucleocapsid protein antibody conjugate with horseradish peroxidase (HRP) onto VLPs, the signal response of the assay was derived from the coulometric measurement of H2O2 reduction mediated by thionine on the electrode surface after the application of a potential (- 0.2 V vs. Ag/AgCl). The ratio of charges measured before or after H2O2 addition was used to quantify IgM because these charges could be used as background charges or total charges, respectively. The ratio exhibited good agreement with IgM concentration within a range 0.1 to 1000 pg mL-1, and a detection limit of 0.06 pg mL-1 was obtained. The assay demonstrated high sensitivity and specificity toward HTNV-specific IgM in serum.

Rapid detection of Pseudomonas syringae pv. actinidiae by electrochemical surface-enhanced Raman spectroscopy

Bacterial cancer caused by Pseudomonas syringae pv. actinidiae (Psa) is a major threat to kiwifruit in the world, and there is still a lack of effective control measures. The field of bacterial detection needs a fast, easy-to-use and sensitive identification platform. The current bacterial identification methods are lack of time efficiency, which brings problems to many sectors of society. Surface-enhanced Raman spectroscopy (SERS) and electrochemistry (EC) have been studied as possible candidates for bacterial detection because of their high sensitivity for the detection of biomolecules. In this work, SERS, EC and electrochemical surface-enhanced Raman spectroscopy (EC-SERS) were used for the first time to study the adsorption and EC behavior of Psa on the surface of nanostructured silver electrodes. Two different Raman spectra of a single analyte were obtained, and this dual detection was realized. Silver nanoparticles with iodide and calcium ions (Ag@ICNPs) were synthesized as SERS substrates significantly enhanced the characteristic signal peaks of Psa, and the limit of detection (LOD) is as low as 1.0 × 102 cfu/mL. Chemical imaging results show that the application of negative voltage can significantly improve the spectrum quality, showing a higher signal at -0.8 V, indicating that Psa molecules may have potential-induced reorientation on the electrode surface. Therefore, EC-SERS has the ability to greatly improve the SERS performance of bacteria in terms of peak intensity and spectral richness.

Bioinspired hot-spot engineering strategy towards ultrasensitive SERS sandwich biosensor for bacterial detection

Surface-enhanced Raman spectroscopy (SERS) sandwich biosensors have received tremendous attention in early diagnosis of bacterial infections. However, efficiently engineering nanoscale plasmonic hots pots (HS) towards ultrasensitive SERS detection still remains challenging. Herein, we propose a bioinspired synergistic HS engineering strategy to construct ultrasensitive SERS sandwich bacterial sensor (named USSB), by coupling bioinspired signal module and plasmonic enrichment module to synergistically boost the number and intensity of HS. The bioinspired signal module is based on dendritic mesoporous silica nanocarrier (DMSN) loaded with plasmonic nanoparticles and SERS tag, while magnetic Fe3O4 nanoparticles coated with Au shell are employed in plasmonic enrichment module. We demonstrate that DMSN effectively shrank nanogaps between plasmonic nanoparticles to improve HS intensity. Meanwhile, plasmonic enrichment module contributed to plenty of additional HS inside and outside individual "sandwich". Ascribing to the boosted number and intensity of HS, the constructed USSB sensor exhibits ultrahigh detection sensitivity (7 CFU/mL) and selectivity towards model pathogenic bacteria of Staphylococcus aureus. Remarkably, the USSB sensor enables fast and accurate bacterial detection in real blood samples of septic mice, achieving early diagnosis of bacterial sepsis. The proposed bioinspired synergistic HS engineering strategy opens up a new direction for constructing ultrasensitive SERS sandwich biosensors, and may promote their advancing applications in the early diagnosis and prognosis of devastating diseases.

Nano-Biotechnology for Bacteria Identification and Potent Anti-bacterial Properties: A Review of Current State of the Art

Sepsis is a critical disease caused by the abrupt increase of bacteria in human blood, which subsequently causes a cytokine storm. Early identification of bacteria is critical to treating a patient with proper antibiotics to avoid sepsis. However, conventional culture-based identification takes a long time. Polymerase chain reaction (PCR) is not so successful because of the complexity and similarity in the genome sequence of some bacterial species, making it difficult to design primers and thus less suitable for rapid bacterial identification. To address these issues, several new technologies have been developed. Recent advances in nanotechnology have shown great potential for fast and accurate bacterial identification. The most promising strategy in nanotechnology involves the use of nanoparticles, which has led to the advancement of highly specific and sensitive biosensors capable of detecting and identifying bacteria even at low concentrations in very little time. The primary drawback of conventional antibiotics is the potential for antimicrobial resistance, which can lead to the development of superbacteria, making them difficult to treat. The incorporation of diverse nanomaterials and designs of nanomaterials has been utilized to kill bacteria efficiently. Nanomaterials with distinct physicochemical properties, such as optical and magnetic properties, including plasmonic and magnetic nanoparticles, have been extensively studied for their potential to efficiently kill bacteria. In this review, we are emphasizing the recent advances in nano-biotechnologies for bacterial identification and anti-bacterial properties. The basic principles of new technologies, as well as their future challenges, have been discussed.

Recent advances in surface enhanced Raman spectroscopy for bacterial pathogen identifications

BACKGROUND

The rapid and reliable detection of pathogenic bacteria at an early stage is a highly significant research field for public health. However, most traditional approaches for pathogen identification are time-consuming and labour-intensive, which may cause physicians making inappropriate treatment decisions based on an incomplete diagnosis of patients with unknown infections, leading to increased morbidity and mortality. Therefore, novel methods are constantly required to face the emerging challenges of bacterial detection and identification. In particular, Raman spectroscopy (RS) is becoming an attractive method for rapid and accurate detection of bacterial pathogens in recent years, among which the newly developed surface-enhanced Raman spectroscopy (SERS) shows the most promising potential.

AIM OF REVIEW

Recent advances in pathogen detection and diagnosis of bacterial infections were discussed with focuses on the development of the SERS approaches and its applications in complex clinical settings.

KEY SCIENTIFIC CONCEPTS OF REVIEW

The current review describes bacterial classification using surface enhanced Raman spectroscopy (SERS) for developing a rapid and more accurate method for the identification of bacterial pathogens in clinical diagnosis. The initial part of this review gives a brief overview of the mechanism of SERS technology and development of the SERS approach to detect bacterial pathogens in complex samples. The development of the label-based and label-free SERS strategies and several novel SERS-compatible technologies in clinical applications, as well as the analytical procedures and examples of chemometric methods for SERS, are introduced. The computational challenges of pre-processing spectra and the highlights of the limitations and perspectives of the SERS technique are also discussed.Taken together, this systematic review provides an overall summary of the SERS technique and its application potential for direct bacterial diagnosis in clinical samples such as blood, urine and sputum, etc.

A SERS bioassay based on vancomycin-modified PEI-interlayered nanocomposite and aptamer-functionalized SERS tags for synchronous detection of Acinetobacter baumannii and Klebsiella pneumoniae

Klebsiella pneumoniae (KP) and Acinetobacter baumannii (AB) are two important gram-negative bacteria that cause pneumonia and have been recently known to be associated with food. The rapid detection of these pathogens in food is important to minimize their colonization of the gut and stop new threats of the disease from spreading across the food chain. Herein, a double-edged sword aptasensor was developed for the synchronous detection of KP and AB in food and clinical samples. A highly sensitive, selective, specific, and synchronous detection of the target bacteria was achieved, and the limit of detection (LOD) was 10 cells/mL with a liner range of 50 to 10⁵ cells/mL. The total assay time was 1.5 h. This study does not only provide a new tool for the detection of the target bacteria, but also serves as a promising tool for food safety and pneumonia diagnosis.

Toward a New Era of SERS and TERS at the Nanometer Scale: From Fundamentals to Innovative Applications

Surface-enhanced Raman scattering (SERS) and tip-enhanced Raman scattering (TERS) have opened a variety of exciting research fields. However, although a vast number of applications have been proposed since the two techniques were first reported, none has been applied to real practical use. This calls for an update in the recent fundamental and application studies of SERS and TERS. Thus, the goals and scope of this review are to report new directions and perspectives of SERS and TERS, mainly from the viewpoint of combining their mechanism and application studies. Regarding the recent progress in SERS and TERS, this review discusses four main topics: (1) nanometer to subnanometer plasmonic hotspots for SERS; (2) Ångström resolved TERS; (3) chemical mechanisms, i.e., charge-transfer mechanism of SERS and semiconductor-enhanced Raman scattering; and (4) the creation of a strong bridge between the mechanism studies and applications.

Characterization of Bacteria Using Surface-Enhanced Raman Spectroscopy (SERS): Influence of Microbiological Factors on the SERS Spectra

SERS is currently being explored as a rapid method for identification of bacteria but variation in the experimental procedures has resulted in considerable variation in the spectra reported for a range of bacterial species. Here, we show that mixing bacteria with a conventional citrate-reduced silver colloid (CRSC) and drying the resulting suspension yield highly reproducible spectra. These signals were due to intracellular components released when the structure of the bacteria was disrupted during sample preparation. This reproducibility allowed us to examine the effects of variables that do not arise in SERS of simple solutions but are relevant in studies of bacteria. These included growth phase and biological variation, which occurred when the same bacterial isolates were cultured under nominally identical conditions on different days. It was found that even under optimal standardized conditions the effect of differences in experimental parameters such as growth phase was very large in some bacterial species but insignificant in others. This suggests that it is important to avoid drawing general conclusions about bacterial SERS based on studies using small numbers of samples. Similarly, discrimination between bacterial species was straightforward when a small number of isolates with distinct spectral features were investigated; however, this became more challenging when more bacterial species were included, as this increased the possibility of finding different species of bacteria with similar spectra. These observations are important because they clearly delineate the challenges that will need to be addressed if SERS is to be used for clinical applications.

Review on combining surface-enhanced Raman spectroscopy and electrochemistry for analytical applications

The discovery of surface enhanced Raman scattering (SERS) from an electrochemical (EC)-SERS experiment is known as a historic breakthrough. Five decades have passed and Raman spectroelectrochemistry (SEC) has developed into a common characterization tool that provides information about the electrode-electrolyte interface. Recently, this technique has been successfully explored for analytical purposes. EC was found to highly improve the performances of SERS sensors, providing, among others, controlled adsorption of analytes and increased reproducibility. In this review, we highlight the potential of EC-SERS sensors to be implemented for point-of-need (PON) analyses as miniaturized devices, and their ability to revolutionize fields like quality control, diagnosis or environmental and food safety. Important developments have been achieved in Raman spectroelectrochemistry, which now represents a promising alternative to conventional analytical methods and interests more and more researchers. The studies included in this review open endless possibilities for real-life EC-SERS analytical applications.

Dual-Approach Electrochemical Surface-Enhanced Raman Scattering Detection of Mycobacterium tuberculosis in Patient-Derived Biological Specimens: Proof of Concept for a Generalizable Method to Detect and Identify Bacterial Pathogens

The recent surge in infectious disease-causing pathogens, resulting in global catastrophe, has merited a pivotal quest toward point-of-care (POC) diagnostics. Mycobacterium tuberculosis (MTB) is still the top bacterium-based infectious disease-causing pathogen worldwide. In a concerted effort toward simplifying and decentralizing the discriminatory screening of MTB causing pathogens, electrochemical surface-enhanced Raman scattering (EC-SERS) was adopted to create a customized screening tool. The development strategy combined five key factors, including (i) a simplified Tollens'-based chemical synthesis method for bulk supply of silver nanoparticles, (ii) the deliberate surface modification of nanoparticles with carefully selected polyelectrolytes to resemble the conditioning layer usually found on a natural substratum, (iii) uniform SERS-active films formed through simple unprogrammed assembly, (iv) the controlled manipulation of the local electric field through applied voltage using a technique that does not conform to the limitations of classical EC-SERS, and (v) the inherent specificity of the target-specific SERS vibrational signature. The EC-SERS platform was able to discriminatively detect and identify TB-derived mycobacteria, including three clinically relevant MTB strains, TB-H37Rv, TB-HN878, and TB-CDC1551. Moreover, a customized voltage stepping protocol, compatible with either the inclusion of a short preincubation step or with in situ EC-SERS is illustrated. From the obtained SERS vibrational signatures, a band indicating a mode unique to TB-derived/TB-affiliated mycobacteria and thus not observed for other bacterial types used in this study was illustrated. Furthermore, provisional investigation, done as prelude for assessing the potential for translational adaptability of the EC-SERS technique toward POC clinical settings for sputum and urine specimens, was carried out.

Organic Anion Detection with Functionalized SERS Substrates via Coupled Electrokinetic Preconcentration, Analyte Capture, and Charge Transfer

Detecting ultralow concentrations of anionic analytes in solution by surface-enhanced Raman spectroscopy (SERS) remains challenging due to their low affinity for SERS substrates. Two strategies were examined to enable in situ, liquid phase detection using 5(6)-carboxyfluorescein (5(6)-FAM) as a model analyte: functionalization of a gold nanopillar substrate with cationic cysteamine self-assembled monolayer (CA-SAM) and electrokinetic preconcentration (EP-SERS) with potentials ranging from 0 to +500 mV. The CA-SAM did not enable detection without an applied field, likely due to insufficient accumulation of 5(6)-FAM on the substrate surface limited by passive diffusion. 5(6)-FAM could only be reliably detected with an applied electric field with the charged molecules driven by electroconvection to the substrate surface and the SERS intensity following the Langmuir adsorption model. The obtained limits of detection (LODs) with an applied field were 97.5 and 6.4 nM on bare and CA-SAM substrates, respectively. For the CA-SAM substrates, both the ligand and analyte displayed an ∼15-fold signal enhancement with an applied field, revealing an additional enhancement due to charge-transfer resonance taking place between the metal and 5(6)-FAM that improved the LOD by an order of magnitude.

Electrochemical surface-enhanced Raman spectroscopy (EC-SERS): a tool for the identification of polyphenolic components in natural lake pigments

The identification of natural organic pigments is important for the conservation, preservation, and historical interpretation of artwork. Due to the fugitive nature of the natural dye components in pigments, their analysis can be complicated by issues such as low concentration and sample complexity. In addition, these pigments are exceedingly diverse, and often represent complex mixtures which are difficult to analyse without a separation step. A particularly challenging class of dyes is the natural yellow polyphenols (i.e. quercetin, rhamnetin, emodin, etc.). Several techniques have been used successfully for the identification of phenolic compounds in a complex mixture, but the majority of these methods require advanced instrumentation and one or more separation steps. In addition, these methods may lack the sensitivity needed to detect minute amounts of pigment remaining in faded artwork. As a result, there is a need for innovative methods of analysis which can be applied to the interpretation of artworks containing natural dyes. In this work, cost-effective screen printed electrodes (SPEs) modified with silver nanoparticles (AgNP) were used to amplify the electrochemical SERS response of phenolic compounds. In particular, application of a voltage to the SERS substrate allows for a fine-tuning of the SERS signal, and was successfully used to separately characterize dye components in two natural yellow lake pigments, Reseda Lake and Stil de Grain. To our knowledge, this work represents the first electrochemical surface-enhanced Raman spectroscopy (EC-SERS) study of polyphenolic dye mixtures, and is the first application of EC-SERS for natural pigment analysis. This work establishes EC-SERS as a useful technique for the identification of complex natural dyes which may find potential use in the cultural heritage realm.

De Novo Approach to Encapsulating Biocatalysts into Synthetic Matrixes: From Enzymes to Microbial Electrocatalysts

Biocatalysts hold great promise in chemical and electrochemical reactions. However, biocatalysts are prone to inhospitable physiochemical conditions. Encapsulating biocatalysts into a synthetic host matrix can improve their stability and activity, and broaden their operational conditions. In this Review, we summarize the emerging de novo approaches to encapsulating biocatalysts into synthetic matrixes. Here, de novo means that embedding of biocatalysts and construction of matrixes take place simultaneously. We discuss the advantages and limitations of the de novo approach. On the basis of the nature of the biocatalysts and the synthetic frameworks, we specifically focus on two aspects: (1) encapsulation of enzymes (in vitro) in metal-organic frameworks and (2) encapsulation of microbial electrocatalysts (in vivo) on the electrode. For both cases, we discuss how the encapsulation improves biocatalysts' performance (stability, viability, activity, and etc.). We also highlight the benefit of encapsulation in facilitating the transport of charge carriers in microbial electrocatalysis.

Dual Functions of a Au@AgNP-Incorporated Nanocomposite Desalination Membrane with an Enhanced Antifouling Property and Fouling Detection Via Surface-Enhanced Raman Spectroscopy

Membrane fouling has remained a major challenge limiting the wide application of membrane technology because it reduces the efficiency and shortens the lifespan of the membrane, thus increasing the operation cost. Herein we report a novel dual-function nanocomposite membrane incorporating silver-coated gold nanoparticles (Au@AgNPs) into a sulfosuccinic acid (SSA) cross-linked poly(vinyl alcohol) (PVA) membrane for a pervaporation desalination. Compared with the control PVA membrane and PVA/SSA membrane, the Au@AgNPs/PVA/SSA membrane demonstrated a higher water flux and better salt rejection as well as an enhanced antifouling property. More importantly, Au@AgNPs provided an additional function enabling a foulant detection on the membrane surface via surface-enhanced Raman spectroscopy (SERS) as Au@AgNPs could amplify the Raman signals as an SERS substrate. Distinct SERS spectra given by a fouled membrane helped to distinguish different protein foulants from their characteristic fingerprint peaks. Their fouling tendency on the membrane was also revealed by comparing the SERS intensities of mixed foulants on the membrane surface. The Au@AgNPs/PVA/SSA nanocomposite membrane presented here demonstrated the possibility of a multifunction membrane to achieve both antifouling and fouling detection, which could potentially be used in water treatment.

Iodide Functionalized Paper-Based SERS Sensors for Improved Detection of Narcotics

An inkjet-printed paper-based Surface-enhanced Raman scattering (SERS) sensor is a robust and versatile device that provides trace sensing capabilities for the detection and analysis of narcotics and drugs. Such sensors generally work well for analytes with good binding affinity towards the Au or Ag plasmonic nanoparticles (NPs) resident in the sensors. In this report, we show that iodide functionalization of the printed sensors helps to remove adsorbed contaminants from AuNP surfaces enabling superior performance with improved detection of narcotics such as fentanyl, heroin and cocaine by SERS. SERS signals are easily doubled with the iodide-functionalized sensors which also showed orders of magnitude improvement in detection limit. In this report, we show that a short (90 s) iodide treatment of the sensors significantly improved the detection of heroin. We propose that iodide functionalization be integrated into field detection kits through the solvent that wets paper-based sensor prior to swabbing for narcotics. Alternatively, we have also demonstrated that iodide functionalized sensors can be stored in ambient for up to 1 week and retain the improved performance towards heroin detection. This report will help to significantly improve the performance of paper-based sensors for field detection of narcotic drugs.

Tuning of Electrode Surface for Enhanced Bacterial Adhesion and Reactions: A Review on Recent Approaches

The study of bacterial adhesion and its consequences has great significance in different fields such as marine science, renewable energy sectors, soil and plant ecology, food industry, and the biomedical field. Generally, the adverse effects of microbial surface interactions have attained wide visibility. However, herein, we present distinct approaches to highlight the beneficial aspects of microbial surface interactions for various applications rather than deal with the conventional negative aspects or prevention strategies. The surface microbial reactions can be tuned for useful biochemical or bio-electrochemical applications, which are otherwise unattainable through conventional routes. In this context, the present review is a comprehensive approach to highlight the basic principles and signature parameters that are responsible for the useful microbial-electrode interactions. It also proposes various surface tuning strategies, which are useful for tuning the electrode characteristics particularly suitable for the enhanced bacterial adhesion and reactions. The tuning of surface characteristics of electrodes is discussed with a special reference to the Microbial Fuel Cell as an example.

Recent advancement in nano-optical strategies for detection of pathogenic bacteria and their metabolites in food safety

Pathogenic bacteria and their metabolites are the leading risk factor in food safety and are one of the major threats to human health because of the capability of triggering diseases with high morbidity and mortality. Nano-optical sensors for bacteria sensing have been greatly explored with the emergence of nanotechnology and artificial intelligence. In addition, with the rapid development of cross fusion technology, other technologies integrated nano-optical sensors show great potential in bacterial and their metabolites sensing. This review focus on nano-optical strategies for bacteria and their metabolites sensing in the field of food safety; based on surface-enhanced Raman scattering (SERS), fluorescence, and colorimetric biosensors, and their integration with the microfluidic platform, electrochemical platform, and nucleic acid amplification platform in the recent three years. Compared with the traditional techniques, nano optical-based sensors have greatly improved the sensitivity with reduced detection time and cost. However, challenges remain for the simple fabrication of biosensors and their practical application in complex matrices. Thus, bringing out improvements or novelty in the pretreatment methods will be a trend in the upcoming future.

A novel SERS substrate with high reusability for sensitive detection of miRNA 21

Electrochemistry combining with Surface Enhanced Raman Spectroscopy (EC-SERS) is a hot area which can achieve real-time analysis of the electrochemical product. In this work, a high-performance reusable Raman substrate was fabricated via electrochemical reduction for miRNA 21 assay. In this strategy, 2'-hydroxymethyl-3, 4-thylenedioxythiophene (EDOT-OH) was electropolymerized to form PEDOT-OH (Red), which acted as a part of SERS substrate and a Raman probe at the same time. The Raman intensity of PEDOT-OH was different between the reduction (Red) and oxidation state (Ox). When it is oxidized, the signal of the PEDOT-OH (Ox) on the electrode surface can be restored by applying a reduction voltage. In view of this feature, a Raman enhanced substrate displaying signal changes is constructed and the constructed Raman substrate can be recycled quickly and efficiently. Combining with double-amplification strategy, the SERS platform can detect miRNA 21 from 100 fM to 1 μM. The Raman substrate can be reused at least 15 times and solves the current problems of poor reusability and troublesome restoration of present reusable Raman substrate. As a result, it indicates that the reusable Raman substrate with high performance and non-destructive property will broaden the application of EC-SERS and SERS analysis.

In situ food-borne pathogen sensors in a nanoconfined space by surface enhanced Raman scattering

The incidence of disease arising from food-borne pathogens is increasing continuously and has become a global public health problem. Rapid and accurate identification of food-borne pathogens is essential for adopting disease intervention strategies and controlling the spread of epidemics. Surface-enhanced Raman spectroscopy (SERS) has attracted increasing interest due to the attractive features including simplicity, rapid measurement, and high sensitivity. It can be used for rapid in situ sensing of single and multicomponent samples within the nanostructure-based confined space by providing molecular fingerprint information and has been demonstrated to be an effective detection strategy for pathogens. This article aims to review the application of SERS to the rapid sensing of food-borne pathogens in food matrices. The mechanisms and advantages of SERS, and detection strategies are briefly discussed. The latest progress on the use of SERS for rapid detection of food-borne bacteria and viruses is considered, including both the labeled and label-free detection strategies. In closing, according to the current situation regarding detection of food-borne pathogens, the review highlights the challenges faced by SERS and the prospects for new applications in food safety.

Graphical abstract

Capillary electrophoretic method for Staphylococcus aureus detection by using a novel antimicrobial peptide

A novel peptide containing antimicrobial sequence and gelatinase cleavage sites was designed for Staphylococcus aureus detection. Since Staphylococcus aureus could secrete gelatinase, the fluorescein labeled peptide GKRWWKWWRRPLGVRGC could be recognized and cleaved. The obtained products were able to be analyzed by capillary electrophoresis with fluorescence detection. To explore the effect of Staphylococcus aureus concentration on enzyme digestion ability of peptide, Staphylococcus aureus with different concentrations were incubated with the peptide. Results indicated that capillary electrophoretic method was efficient for determining Staphylococcus aureus content. Compared with traditional approaches for Staphylococcus aureus detection, capillary electrophoresis possessed higher efficiency, enhanced sensitivity, and low sample consumption. Moreover, the proposed peptide also presented desirable antimicrobial activity. It suggested that the novel antimicrobial peptide used in this research opens a new path of detecting Staphylococcus aureus by capillary electrophoretic method.

Effective Electrochemical Modulation of SERS Intensity Assisted by Core-Shell Nanoparticles

An effective and reversible tuning of the intensity of surface-enhanced Raman scattering (SERS) of nonelectroactive molecules at nonresonance conditions by electrochemical means has been developed on plasmonic molecular nanojunctions formed between Au@Ag core-shell nanoparticles (NPs) and a gold nanoelectrode (AuNE) modified with a self-assembled monolayer. The Au@Ag nanoparticle on nanoelectrode (NPoNE) structures are formed in situ by the electrochemical deposition of Ag on AuNPs adsorbed on the AuNE and can be monitored by both the electrochemical current and SERS signals. Instead of introducing molecular changes by the applied electrode potential, the highly effective SERS intensity tuning was achieved by the chemical composition transformation of the ultrathin Ag shell from metallic Ag to insulating AgCl. The electrode potential-induced electromagnetic enhancement (EME) tuning in the Au@Ag NPoNE structure has been confirmed by finite-difference time-domain simulations. Moreover, the specific Raman band associated with Ag-molecule interaction can also be tuned by the electrode potential. Therefore, we demonstrated that the electrode potential could effectively and reversibly modulate both EME and chemical enhancement in Au@Ag NPoNE structures.

Integrated EC-SERS Chip with Uniform Nanostructured EC-SERS Active Working Electrode for Rapid Detection of Uric Acid

Toxemia of pregnancy is a very dangerous disease for pregnant women. The mortality rate of toxemia of pregnancy is close to 10% to 15%. Early detection of pregnancy toxemia is to monitoring uric acid concentration in urine. The current mainstream method for detecting uric acid requires an enzyme (urate oxidase), which needs to be stored in a low-temperature environment, and the method requires complex chemical steps, which takes a longer time and more samples. In this study, we propose an integrated miniature three-electrode electrochemical surface-enhanced Raman spectroscopy chip (EC-SERS chip) suitable for rapid EC-SERS detection applications. The integrated microfluidic reservoir on the chip makes it easy to use, which is very suitable for rapid detection applications. The SERS active working electrode for the proposed integrated EC-SERS chip is a nanocone array polycarbonate (PC) substrate decorated with an evenly distributed and tightly packed array of gold nanospheres. It showed good uniformity and can be easily reproduced. The integrated EC-SERS chip is very small compared to the traditional electrochemical cell, which reduces the sample volume required for the testing. In addition, the chip is for one-time use only. It eliminates the need to clean electrochemical cells for reuse, thereby reducing the possibility of contamination and inaccurate detection. Various low-concentration Rhodamine 6G (R6G) solutions were tested to verify the performance of the developed EC-SERS chip. Experimental results showed that the proposed EC-SERS chip has a strong enhancement factor of up to 8.5 × 10⁶ and a very good EC-SERS uniformity (the relative standard deviation of EC-SERS intensity is as low as 1.41%). The EC-SERS chip developed has been further tested for the detection of uric acid in synthetic urine. The results showed that the EC-SERS signal intensity has a highly linear relationship with the logarithm of the uric acid concentration in synthetic urine, which indicates that the developed EC-SERS chip is suitable for the quantitative detection of uric acid in synthetic urine. Therefore, the developed EC-SERS chip is very promising to be used in routine and early diagnosis of pregnancy toxemia and may be used in many other medical tests, food safety, and biotechnology applications.

Bacteria Detection: From Powerful SERS to Its Advanced Compatible Techniques

The rapid, highly sensitive, and accurate detection of bacteria is the focus of various fields, especially food safety and public health. Surface-enhanced Raman spectroscopy (SERS), with the advantages of being fast, sensitive, and nondestructive, can be used to directly obtain molecular fingerprint information, as well as for the on-line qualitative analysis of multicomponent samples. It has therefore become an effective technique for bacterial detection. Within this progress report, advances in the detection of bacteria using SERS and other compatible techniques are discussed in order to summarize its development in recent years. First, the enhancement principle and mechanism of SERS technology are briefly overviewed. The second part is devoted to a label-free strategy for the detection of bacterial cells and bacterial metabolites. In this section, important considerations that must be made to improve bacterial SERS signals are discussed. Then, the label-based SERS strategy involves the design strategy of SERS tags, the immunomagnetic separation of SERS tags, and the capture of bacteria from solution and dye-labeled SERS primers. In the third part, several novel SERS compatible technologies and applications in clinical and food safety are introduced. In the final part, the results achieved are summarized and future perspectives are proposed.

Surface Functional Nanofiber Membrane for Ultrasensitive and Naked-Eye Visualization of Bacterial Concentration

Bacterial contamination in water is a serious health risk to human beings, so it is very important to realize the point-of-care (POC) bacterial detection in water. However, the traditional bacterial detection methods are time-consuming, professional- and equipment-dependent, and do not meet the needs of POC detection. There is a pressing need to develop a platform for POC bacterial detection to defeat the increasing risk of bacterial infections. Herein, a surface functional nanofiber membrane (NFM) is prepared by layer-by-layer (LBL) self-assembly as a platform for POC detection of bacterial concentration; it is naked-eye visualization and ultrasensitive. The platform shows obvious bacterial responsiveness, which allows naked-eye visualization of bacterial concentration (10²-10⁶ CFU/mL) within 30 min and can quantitatively detect the bacterial concentration (10¹-10⁶ CFU/mL) by fluorescence within 5 min. The platform not only exhibits high efficiency but also has a low threshold for bacterial concentration detection. Furthermore, the platform shows good consistency with traditional methods in the detection of bacteria in practical water samples, and has the potential for use in detecting bacterial concentrations in water supplies to protect human beings from health hazards. This work also provides useful reference for research on bacterial detection, taking advantage of the surface characteristics of bacteria and the high sensitivity of NFM.

Electrochemical SERS on 2D Mapping for Metabolites Detection

Surface-enhanced Raman scattering (SERS) has been widely used for bioanalysis because it provides a high sensitivity for detecting analytes of ultralow concentrations. However, the clinical application of a 2D SERS-active substrate remains challenging because of the difficulty of obtaining accurate quantification, especially at low concentration. In this study, we proposed an analytical method that integrates an optimized sample mapping strategy with an electrochemical SERS (EC-SERS) technique to resolve this problem. We adopted this method to detect two metabolites of azathioprine, namely 6-thioguanine nucleotides (6-TGNs) and 6-methylmercaptopurine (6-MMP), as our proof-of-concept experiment. We first prepared a conductive SERS-active substrate by electrochemically depositing Au nanoparticles (AuNPs) on indium tin oxide glass. The two metabolites were then randomly absorbed on the surface of the AuNPs of the SERS-active substrates. When we applied a negative potential on the substrate, we observed a large enhancement of Raman intensity for both metabolites, which was attributed to both the charge transfer effect and reorientation of metabolites on the substrate surface, leading to the formation of Au-S bonds. In addition, by optimizing the mapping range, we were able to efficiently reduce the standard deviation of SERS intensity and achieve a consistent standard deviation lower than 10%. With these two features, we were able to achieve quantitative analysis of 6-TGNs and 6-MMP with a detection limit of 10 and 100 nM, respectively. The integration of EC-SERS and the mapping method provided a reliable and quantitative analytical platform for analytes, which can be electrochemically modulated, like 6-TGNs and 6-MMP.

A Self-Calibrating Surface-Enhanced Raman Scattering-Active System for Bacterial Phenotype Detection

Pathogen detection is of significant importance in human health and safety due to the high morbidity and mortality induced by bacterial infections. Therefore, the development of rapid, sensitive, and selective methods for the discrimination of pathogens is the key to improve the patient survival rates. In this work, we develop a new self-calibrating surface-enhanced Raman scattering (SERS)-based sensor that enables sensitive and reproducible pathogen detection in practical samples. The assay makes use of gold nanoflowers (AuNFs) consisting of three components: a solid Au core of ∼15 nm, a hollow gap of ∼1 nm, and a flower-like Au shell. We have demonstrated that the sensitive and quantitative analysis of biomolecules can be achieved by the target-dependent, sequence-specific DNA hybridization assembly between AuNFs with a built-in internal standard. We further demonstrate that this kind of reliable SERS sensor is able to distinguish different bacteria with sensitivity down to single bacterium. We expect that the established quantitative SERS technique could provide a promising tool for widespread applications in biomedical research and clinical diagnostics.

Developing a toll-like receptor biosensor for Gram-positive bacterial detection and its storage strategies

Conditions to store toll-like receptor2/6 sensors and use them to detect bacterial analytes, including pathogen-associated molecular patterns and bacterial cultures.

Parameter Selection for a Microvolume Electrochemical Escherichia coli Detector for Pairing with a Concentration Device

Waterborne infections are responsible for health problems worldwide and their prompt and sensitive detection in recreational and potable water is of great importance. Bacterial identification and enumeration in water samples ensures water is safe for its intended use. Culture-based methods can be time consuming and are usually performed offsite. There is a need to for automated and distributed at-source detectors for water quality monitoring. Herein we demonstrate a microvolume Escherichia coli (E. coli) detector based on a screen printed electrode (SPE) bioelectroanalytical system and explore to what extent performance can be improved by coupling it with a filtration device. To confidently benchmark detector performance, we applied a statistical assessment method to target optimal detection of a simulated concentrated sample. Our aim was to arrive at a holistic understanding of device performance and to demonstrate system improvements based on these insights. The best achievable detection time for a simulated 1 CFU mL-1 sample was 4.3 (±0.6) h assuming no loss of performance in the filtration step. The real filtered samples fell short of this, extending detection time to 16-18 h. The loss in performance is likely to arise from stress imposed by the filtration step which inhibited microbial growth rates.