Flexible fabrication of a paper-fluidic SERS sensor coated with a monolayer of core–shell nanospheres for reliable quantitative SERS measurements

Plasmonic Nanostructures Grown from Reacting Droplet-In-Microwell Array on Flexible Films for Quantitative Surface-Enhanced Raman Spectroscopy in Plant Wearable In Situ Detection

Plant wearable detection has garnered significant interest in advancing agricultural intelligence and promoting sustainable food production amidst the challenges of climate change. Accurately monitoring plant health and agrochemical residue levels necessitates qualities such as precision, affordability, simplicity, and noninvasiveness. Here, a novel attachable plasmonic film is introduced and designed for on-site detection of agrochemical residues utilizing surface-enhanced Raman spectroscopy (SERS). By functionalizing a thin polydimethylsiloxane film with silver nanoparticles via controlled droplet reactions in micro-well arrays, a plasmonic film is achieved that not only maintains optical transparency for precise analyte localization but also conforms closely to the plant surface, facilitating highly sensitive SERS measurements. The reliability of this film enables accurate identification and quantification of individual compounds and their mixtures, boasting an ultra-low detection limit ranging from 10-16 to 10-13 m, with mini mal relative standard deviation. To showcase its potential, on-field detection of pesticide residues on fruit surfaces is conducted using a handheld Raman spectrometer. This advancement in fabricating plasmonic nanostructures on flexible films holds promise for expanding SERS applications beyond plant monitoring, including personalized health monitoring, point-of-care diagnosis, wearable devices for human-machine interface, and on-site monitoring of environmental pollutants.

An in-situ method for SERS substrate preparation and optimization based on galvanic replacement reaction

BACKGROUND

Various surface-enhanced Raman spectroscopy (SERS) substrate preparation methods have been reported, however, how to tune the "gap" between nanostructures to make more "hot spots" is still a barrier that restricts their application. The gap between nanostructures is usually fixed when the substrates are prepared. In other words, it is hard to tune interparticle distances for maximum electromagnetic coupling during substrate preparation process. Therefore, an in-situ substrate optimization method that could monitor the SERS signal intensity changes, i.e., to find the optimum gap width and particle size, during substrate preparation process is needed.

RESULTS

A method based on the galvanic replacement reaction (GRR) is proposed for the in-situ gap width tuning between nanostructures as well as for the optimization of SERS substrates. Noble metal nanoparticles (NPs) form and grow on the sacrificial templates' surface while noble metal ions are reduced by sacrificial metal (oxides) in GRR. Along with the fresh and clean NPs' surface generated, the gap between two noble metal NPs decreases with the growth of the NPs. To demonstrate this strategy, cuprous oxide/Ti (Cu2O/Ti) sacrificial templates were prepared, and then a GRR was carried out with HAuCl4. The real-time SERS detection during GRR show that the optimum reaction time (ORT) is 300 ± 30 s. Furthermore, SERS performance testing was conducted on the optimized substrate, revealing that the detection limit for crystal violet can reach 1.96 × 10-11 M, confirming the feasibility of this method.

SIGNIFICANCE AND NOVELTY

By monitoring the in-situ SERS signal of probes during GRR will obtain an "optimal state" of the SERS substrate with optimal gap width and particle size. The SERS substrate preparation and optimization strategy proposed in this article not only provides a simple, efficient, and low-cost method to fabricate surface-clean noble NPs but also paves the way for the in-situ optimization of NPs size and gap width between NPs which could achieve wider applications of SERS.

Ratiometric SERS quantification of SO2 vapor based on Au@Ag-Au with Raman reporter as internal standard

Practical gas sensing application requires sensors to quantify target analytes with high sensitivity and reproducibility. However, conventional surface enhanced Raman scattering (SERS) sensor lacks reproducibility and quantification arising from variations of "hot spot" distribution and measurement conditions. Here, a ratio-dependent SERS sensor was developed for quantitative label-free gas sensing. Au@Ag-Au nanoparticles (NPs) were filtered onto anodic aluminum oxide (AAO) forming Au@Ag-Au@AAO SERS substrate. 4-MBA was encapsulated in the gap of Au@Ag-Au and served as the internal standard (IS) to calibrate SERS signal fluctuation for improved quantification ability. Combined with headspace sampling method, SO2 residue in traditional Chinese medicine (TCM) can be extracted and captured on the immediate vicinity of Au@Ag-Au surface. The intensity ratio I613 cm-1/I1078 cm-1 showed excellent linearity within the range of 0.5 mg/kg-500 mg/kg, demonstrating superior quantification performance for SO2 detection. Signals for concentration as low as 0.05 mg/kg of SO2 could be effectively collected, much lower than the strictest limit 10 mg/kg in Chinese Pharmacopoeia. Combined with a handheld Raman spectrometer, handy and quantitative TCM quality evaluation in aspect of SO2 residue was realized. This ratiometric SERS sensor functioned well in rapid on-site SO2 quantification, exhibiting excellent sensitivity and simple operability.

Recent developments and applications of surface enhanced Raman scattering spectroscopy in safety detection of fruits and vegetables

This article reviewed the latest research progress of Surface-enhanced Raman Spectroscopy (SERS) in the security detection of fruits and vegetables in recent years, especially in three aspects: pesticide residues, microbial toxin contamination and harmful microorganism infection. The binding mechanism and application potential of SERS detection materials (including universal type and special type) and carrier materials (namely rigid and flexible materials) were discussed. Finally, the application prospect of SERS in fruit and vegetable safety detection was explored, and the problems to be solved and development trends were put forward. The poor stability and reproducibility of SERS substrates make it difficult for practical applications. It is necessary to continuously optimize SERS substrates and develop small and portable Raman spectroscopy analyzers. In the future, SERS technology is expected to play an important role in human health, food safety and economy.

Soft Epidermal Paperfluidics for Sweat Analysis by Ratiometric Raman Spectroscopy

The expanding interest in digital biomarker analysis focused on non-invasive human bodily fluids, such as sweat, highlights the pressing need for easily manufactured and highly efficient soft lab-on-skin solutions. Here, we report, for the first time, the integration of microfluidic paper-based devices (μPAD) and non-enhanced Raman-scattering-enabled optical biochemical sensing (Raman biosensing). Their integration merges the enormous benefits of μPAD, with high potential for commercialization and use in resource-limited settings, with biorecognition-element-free (but highly selective) optical Raman biosensing. The introduced thin (0.36 mm), ultra-lightweight (0.19 g), and compact footprint (3 cm2) opto-paperfluidic sweat patch is flexible, stretchable, and conforms, irritation-free, to hairless or minimally haired body regions to enable swift sweat collection. As a great advantage, this new bio-chemical sensory system excels through its absence of onboard biorecognition elements (bioreceptor-free) and omission of plasmonic nanomaterials. The proposed easy fabrication process is adaptable to mass production by following a fully sustainable and cost-effective process utilizing only basic tools by avoiding typically employed printing or laser patterning. Furthermore, efficient collection and transportation of precise sweat volumes, driven exclusively by the wicking properties of porous materials, shows high efficiency in liquid transportation and reduces biosensing latency by a factor of 5 compared to state-of-the-art epidermal microfluidics. The proposed unit enables electronic chip-free and imaging-less visual sweat loss quantification as well as optical biochemical analysis when coupled with Raman spectroscopy. We investigated the multimodal quantification of sweat urea and lactate levels ex vivo (with syntactic sweat including +30 sweat analytes on porcine skin) and achieved a linear dynamic range from 0 to 100 mmol/L during fully dynamic continuous flow characterization.

Ag triangle nanoplates assembled on PVC/SEBS membrane as flexible SERS substrates for skin cortisol sensing

Surface-enhanced Raman scattering (SERS) based on rigid substrates has been widely used in biomedical detection due to its high sensitivity and specificity. However, the tedious operation steps for preparing SERS rigid substrates limited their applications in real-world detection. Compared with general rigid substrate, the flexible substrate has the advantages of simple preparation and easy portability, which are suitable for rapid, wearable and personalized detection in the field of point-of-care test. Herein, the flexible SERS substrates employing copolymer were fabricated and used for detection of skin cortisol, a biomarker for evaluating psychological stress in sweat. Silver triangle nanoplates with sharp corner were used as enhanced particles, and transferred to polyvinyl chloride/styrene-ethylene-butene-styrene copolymer (PVC/SEBS) film through three-phase interface self-assembly. By adjusting the size of silver nanoparticles, the ratio of PVC to SEBS in the polymer film, and the number of transfers of self-assembled silver films, the enhancement effect of the flexible SERS substrate was maximized. In addition, functionalization of the flexible SERS substrate with cortisol antibodies is used to achieve specific detection of cortisol on the skin surface. Under the optimal conditions, the Raman peak intensities at 1268 and 1500 cm-1 of the cortisol had a good linear relationship with the logarithm of its concentration in the range of 10-7 to 10-3 M, and the detection limits were 5.47 × 10-8 M and 5.51 × 10-8 M, respectively. The flexible silver triangle nanoplates SERS substrate constructed by self-assembly in the three-phase interface has the characteristics of good specificity and high sensitivity, which has potential for transdermal cortisol wearable detection, providing a feasible method for the rapid evaluating psychological stress level.

Paper-based substrates for surface-enhanced Raman spectroscopy sensing

Surface-enhanced Raman scattering (SERS) has been recognized as one of the most sensitive analytical methods by adsorbing the target of interest onto a plasmonic surface. Growing attention has been directed towards the fabrication of various substrates to broaden SERS applications. Among these, flexible SERS substrates, particularly paper-based ones, have gained popularity due to their easy-to-use features by full contact with the sample surface. Herein, we reviewed the latest advancements in flexible SERS substrates, with a focus on paper-based substrates. Firstly, it begins by introducing various methods for preparing paper-based substrates and highlights their advantages through several illustrative examples. Subsequently, we demonstrated the booming applications of these paper-based SERS substrates in abiotic and biological matrix detection, with particular emphasis on their potential application in clinical diagnosis. Finally, the prospects and challenges of paper-based SERS substrates in broader applications are discussed.

"On-site" analysis of pesticide residues in complex sample matrix by plasmonic SERS nanostructure hybridized hydrogel

BACKGROUND

Surface-enhanced Raman spectroscopy (SERS) has been extensively used in biomedical and food safety detection due to its advantages of label-free, in situ and fingerprint spectrum. However, it is challenging to develop an excellent SERS substrate that possesses all three of these characteristics including sensitivity, repeatability and stability.

RESULTS

In this work, a specific sodium alginate hydrogel flexible SERS substrate encapsulated gold-silver core-shell nanoparticles (Au@Ag NPs) was developed to address the aforementioned issue. The Au@Ag NPs with SERS "hot spot" structure were evenly dispersed in the hydrogel, which achieved the direct and high efficiency detection of the pesticide residues from complex sample matrix. Taking thiram as objective, this SERS substrates exhibit high sensitivity (detection limit of approximately 1 × 10-10 mol/L), excellent stability (maintain above 78.35 % of SERS activity after 7 weeks) and outstanding repeatability (RSD in one substrate as low as 3.56 %). Furthermore, the flexible hydrogel SERS substrates can be used to analyze a variety of small molecules in real samples (juices, vegetables and fruits), without the need for a laborious pretreatment process.

SIGNIFICANCE

In light of the aforementioned benefits, the functional flexible hydrogel SERS substrates present a reliable platform for the accurate and on-site measurement of chemical contaminants from complex samples.

Fabrication of flexible SERS substrate based on Au nanostars and PDMS for sensitive detection of Thiram residue in apple juice

We developed a novel fabrication of flexible surface-enhanced Raman scattering (SERS) substrate to perform selective and sensitive determination of thiram residue in fruits and juices. Au nanostars (Au NSs) with multi-branching structure were self-assembled on aminated Polydimethylsiloxane (PDMS) slides by electrostatic interaction. By measuring the Thiram's characteristic peak intensity at 1371 cm^-1, the SERS method could distinguish Thiram from other pesticide residues. A good linear relationship between the peak intensity at 1371 cm^-1 and thiram's concentration was established at the range from 0.01 ppm to 100 ppm and the Limit of detection is 0.0048 ppm. We directly used this SERS substrate to detect Thiram in apple juice. By standard addition method, recoveries varied in the range of 97.05% to 106.00% and the RSD were from 3.26% to 9.35%. The SERS substrate exhibited a good sensitivity, stability and selectively for the detection of Thiram in food samples, which can be spread as a common method for the detection of pesticides in food samples.

Regulated synthesis of an Au NB-DT@Ag bimetallic core-molecule-shell nanostructure for reliable SERS detection

In recent research, anisotropic plasmonic core-shell nanomaterials have gained a lot of attention in surface-enhanced Raman scattering (SERS) due to their brilliant uniformity and optical properties. Herein, a bimetallic core-molecule-shell (CMS) composite nanorod SERS substrate nanomaterial (Au NB-DT@Ag NRs) was designed and synthesized under precise regulation. The inner core is gold nanobipyramids (Au NBs), which possess superior plasmonic properties. Uniform Au NBs of five different sizes were fabricated via a penta-twinned gold seed mediated growth method. The length varied from 160 to 62 nm and the corresponding diameter varied from 60 to 23 nm while the longitudinal surface plasmonic resonance (SPR) changed from 908 to 715 nm. The SERS activity of five Au NBs were compared and the optimally sized one with a length of 78 nm and width of 28 nm was set as the inner core. After modification with the Raman reporter (DT), different amounts of silver were deposited on the surface of Au NB-DTs to form an Au NB-DT@Ag nanocomposite. The shape of the nanostructure gradually became a rod and lengthened while the longitudinal SPR wavelength varied from 733 nm to 664 nm with an increase in the amount of silver nitrate added. The Au NB-DT@Ag NRs with the best SERS activity (b-3) could realize the quantitative detection of the toxic dyes malachite green (MG) and crystal violet (CV) of concentrations as low as 5 × 10-9 M, showing good reproducibility and stability. This work offers a new design strategy for a SERS substrate for reliable quantitative SERS detection applications.

Scalable fabrication of silver covered polyurethane nanofibers as flexible SERS nanosensors for aflatoxin detection

Flexible surface enhanced Raman spectroscopy (SERS) nanosensors, constructed by integration of plasmonic nanostructures with polymeric substrates, have received increasing research interests for recent decades. When compared to abundant works on optimization of the plasmonic nanostructures, the research involving the influence of polymeric substrates on analytical performance of resultant flexible SESR nanosensors is unexpectedly limited. Herein, the ultra-thin silver layer has been deposited on the electrospun polyurethane (ePU) nanofibrous membranes via vacuum evaporation to prepare the flexible SRES nanosensors. Interestingly, we find that the molecular weight and polydispersion index of synthesized PU play important roles in regulating the fine morphology of electrospun nanofibers, which in turns determine the Raman enhancement of resultant flexible SERS nanosensors. Specifically, the optimized SERS nanosensor, obtained by evaporating 10 nm silver layer on top of nanofibers derived from electrospinning of PU with a weight-average molecular weight of 140,354 and polydispersion index of 1.26, enables label-free detection of the carcinogen of aflatoxin down to 0.1 nM. Thanks to its scalable fabrication and good sensitivity, the current work opens new way for design of cost-effective flexible SERS nanosensors for environmental monitoring and food security applications.

Nondestructive testing methods for pesticide residue in food commodities: A review

Pesticides play an important role in increasing the overall yield and productivity of agricultural foods by controlling pests, insects, and numerous plant-related diseases. However, the overuse of pesticides has resulted in pesticide contamination of food products and water bodies, as well as disruption of ecological and environmental systems. Global health authorities have set limits for pesticide residues in individual food products to ensure the availability of safe foods in the supply system and to assist farmers in developing the best agronomic practices for crop production. Therefore, the use of nondestructive testing (NDT) methods for pesticide residue detection is gaining interest in the food supply chain. The NDT techniques have several advantages, such as simultaneous measurement of chemical and physical characteristics of food without destroying the product. Although numerous studies have been conducted on NDT for pesticide residue in agro-food products, there are still challenges in real-time implementation. Further study on NDT methods is needed to establish their potential for supplementing existing methods, identifying mixed pesticides, and performing volumetric quantification (not surface accumulation alone).

Cost-Effective and Facile Fabrication of a Tattoo Paper-Based SERS Substrate and Its Application in Pesticide Sensing on Fruit Surfaces

Surface-enhanced Raman spectroscopy (SERS) has been transformed into a useful analytical technique with significant advantages in relation to sensitive and low-concentration chemical analyses. However, SERS substrates are expensive and the analyte sample preparation is complicated; hence, it is only used in limited areas. We have fabricated a tattoo paper-based SERS substrate by using non-complicated inkjet printing. The sensitivity of the SERS substrate was increased by removing the carbon residues via exposure to ultraviolet light without damaging the substrate. Thus, low concentrations of pesticides (up to 1 μM thiram) were measured. The SERS substrate was attached to the curved surface of an apple to demonstrate its advantages, such as the flexibility and easy attachability of tattoo paper, and its feasibility was verified by measuring 1 μM thiram on the apple's surface. Due to its economic cost, simple usage, and rapid measurement, it will be helpful for the identification of both agricultural adulterants and food adulterants and for water-based pollutant detection. It will also possibly be helpful for medical purposes related to human body surfaces in the future.

Facile fabrication of flexible AuNPs@CDA SERS substrate for enrichment and detection of thiram pesticide in water

Pesticide residues in water is one of the most serious problems in developing countries. Surface enhanced Raman spectroscopy (SERS) is widely used in the detection and monitoring of pesticide and other trace compounds because of its low limits of detection (LODs). However, different SERS substrate synthesis methods have different economic benefits and environmental impacts. In this paper, a flexible AuNPs@CDA SERS substrate was fabricated by the gold nanoparticles (AuNPs) and the biomass-based cellulose diacetate (CDA), which had stable test performance and considerable LODs. The substrates were economically viable and environment friendly. The characterization analysis of the substrate allows us to flexibly select different test methods (drop-test or enrichment-test) as coping strategy in different situation. The results showed that the LODs of thiram pesticide in water by enrichment-test could reach 10^-7 g/mL, and had a good linear relationship in the concentration range of 10^-7-10^-6 g/mL. This strategy can realize the rapid and effective detection and monitoring of thiram pesticide in water.

Preparation of SERS active filter paper for filtration and detection of pesticides residue from complex sample

The selectivity is needed mostly in SERS sensing because analytes of interest are commonly present in a complex mixture containing particles and impurities, which hinder the interactions between the laser and analyte being detected. In this manuscript, we describe our efforts developing a simple and instant. method to prepare a filter paper SERS sensor. Colloidal Ag nanoparticles were immobilized on one side of filter paper via an in-situ growth method. The fabrication process of the sensor could be finished in several minutes, and no special facility needed. The filter paper SERS sensor demonstrated a spectra uniformity with a 7.0 % point-by-point signal deviation. And the filter function of the sensor could effectively filter out interferences from samples in 1 min, that allowing the direct detection of thiram in ketchup by SERS with detection limit of 93 ppb. Furthermore, we used a Quick Easy Cheap Effective Rugged and Safe (QuEChERS) sample preparation method to detect malachite green (MG) in soil with a sensitivity as low as 0.01 ppm without any sample pre-treatment or purification. A SERS filter paper may open a new avenue for rapid testing of food quality during manufacturing as well as fast detection of potential contaminants in a myriad of substrates.

Quantitative SERS sensing mediated by internal standard Raman signal from silica nanoparticles in flexible polymer matrix

Attributed to poor signal uniformity and external interference, ultrasensitive surface-enhanced Raman spectroscopy (SERS) still faces difficulties in the reliable and quantitative detection of trace molecules. Here, a facile Ag/Si/sodium carboxy methyl cellulose (NaCMC) film with internal standard (IS) was promoted for quantitative determination of thiram. The effects of preparation conditions on SERS activity of the film were systematically investigated and then a flexible SERS substrate with high sensitivity and uniformity was fabricated. The enhancement factor was calculated to be 1.12 × 10⁶ and SERS mapping was recorded with a relative standard deviation value of 19.8% by utilizing 4-mercaptobenzoic acid (4-MBA) as target molecule. Additionally, the dominant contribution of the IS from encapsulated Si nanoparticles (NPs) was confirmed in the quantitative assay of 4-MBA and thiram, facilitating attractive fitting coefficients (R²) as 0.991 and 0.998. Besides that, the proposed flexible film was conducted to scrub trace thiram from the surfaces of apple, orange, and cucumber, resulting in recoveries of 89%, 94%, and 91%. A smart and facile quantitative SERS substrate was developed here for monitoring trace biochemical molecules, verifying its potential utilizations in monitoring pesticide residues.

Flexible surface-enhanced Raman scatting substrates: recent advances in their principles, design strategies, diversified material selections and applications

Surface-enhanced Raman scattering (SERS) is widely used as a powerful analytical technology in cutting-edge areas such as food safety, biology, chemistry, and medical diagnosis, providing ultra-fast, ultra-sensitive, nondestructive characterization and achieving ultra-high detection sensitivity even down to the single-molecule level. Development of Raman spectroscopy is strongly dependent on high-performance SERS substrates, which have long evolved from the early days of rough metal electrodes to periodic nanopatterned arrays building on solid supporting substrates. For rigid SERS substrates, however, their applications are restricted by sophisticated pretreatments for detecting solid samples with non-planar surfaces. It is therefore essential to reassert the principles in constructing flexible SERS substrates. Herein, we comprehensively review the state-of-the-art in understanding, preparing and using flexible SERS. The basic mechanisms behind the flexible SERS are briefly outlined, typical design strategies are highlighted and diversified selection of materials in preparing flexible SERS substrates are reviewed. Then the recent achievements of various interdisciplinary applications based on flexible SERS substrates are summarized. Finally, the challenges and perspectives for future evolution of flexible SERS and their applications are demonstrated. We propose new research directions focused on stimulating the real potential of SERS as an advanced analytical technique for commercialization.

In Situ Collection and Rapid Detection of Pathogenic Bacteria Using a Flexible SERS Platform Combined with a Portable Raman Spectrometer

This study aims to develop a simple, sensitive, low-cost, environmentally friendly and flexible surface-enhanced Raman scattering (SERS) platform, combined with a portable Raman spectrometer, for the rapid and on-site SERS detection of bacteria. Commercial tobacco packaging paper (TPP) with little background interference was used as a loading medium that effectively adsorbed Au nanoparticles and provided sufficient “hot spots”. This Au-tobacco packaging paper (Au-TPP) substrate used as a flexible SERS platform can maximize sample collection by wiping irregular surfaces, and was successfully applied to the on-site and rapid detection of pathogenic bacteria. Raman fingerprints of pathogenic bacteria can be obtained by SERS detection of spiked pork using wipeable Au-TPP, which verifies its value in practical applications. The results collected by SERS were further verified by polymerase chain reaction (PCR) results. It showed several advantages in on-site SERS detection, including accurate discrimination, simple preparation, easy operation, good sensitivity, accuracy and reproducibility. This study indicates that the established flexible SERS platform has good practical applications in pathogenic bacterial identification and other rapid detections.

Flexible membrane composite based on sepiolite/chitosan/(silver nanoparticles) for enrichment and surface-enhanced Raman scattering determination of sulfamethoxazole in animal-derived food

A sepiolite/chitosan/silver nanoparticles (Sep/CTs/AgNPs) membrane substrate has been developed for the fast separation, enrichment, and surface-enhanced Raman scattering (SERS) determination of sulfamethoxazole all-in-one. The Sep/CTs/AgNPs membrane substrate possessed the ability of rapid separation and enrichment to simplify the process for pretreatment and improve the efficiency of analysis. The grown AgNPs can provide abundant hot spots and plasmonic areas to amplify the Raman signals of target molecules effectively. The membrane substrate exhibited good stability with relative standard deviations of 5.8% and 7.1% to same batch and different batches membrane substrate, respectively,  by detecting sulfamethoxazole. The SERS method based on Sep/CTs/AgNPs membrane substrate was used for the determination of sulfamethoxazole with a linear range of 0.05-2.0 mg/L, and the limit of detection was 0.020 mg/L. The established SERS method was finally applied to the quantification of sulfamethoxazole in animal-derived food samples. Sulfamethoxazole was actually found in crucian sample with 12.4 μg/kg, and the result was confirmed by a high-performance liquid chromatography method with relative error of 5.3%. The whole process of analysis can be finished within 25 min with recoveries of 89.3-102.2%. The SERS method based on Sep/CTs/AgNPs membrane substrate provided an integrated strategy for rapid and accurate SERS analysis in food safety issues.

Ultrasensitive detection of plant hormone abscisic acid-based surface-enhanced Raman spectroscopy aptamer sensor

Abscisic acid (ABA), as the most common plant hormone in the growth of wheat, can greatly affect the yield when its levels deviate from normal. Therefore, highly sensitive and selective detection of this hormone is greatly needed. In this work, we developed an aptamer sensor based on surface-enhanced Raman spectroscopy (SERS) and applied it for the high sensitivity detection of ABA. Biotin-modified ABA aptamer complement chains were modified on ferrosoferric oxide magnetic nanoparticles (Fe₃O₄MNPs) and acted as capture probes, and sulfhydryl aptamer (SH-Apt)-modified silver-coated gold nanospheres (Au@Ag NPs) were used as signal probes. Through the recognition of the ABA aptamer and its complementary chains, an aptamer sensor based on SERS was constructed. As SERS internal standard molecules of 4-mercaptobenzoic acid (4-MBA) were encapsulated between the gold core and silver shell of the signal probes; the constructed aptamer sensor generated a strong SERS signal of 4-MBA after magnetic separation. When there were ABA molecules in the detection system, with the preferential binding of ABA aptamer and ABA molecule, the signal probes were released from the capture probes, after magnetic separation, leading to a linear decrease in SERS intensity of 4-MBA. Thus, the detection response was linear over a logarithmic concentration range, with an ultra-low detection limit of 0.67 fM. In addition, the practical use of this assay method was demonstrated in ABA detection from fresh wheat leaves, with a relative error (RE) of 5.43-8.94% when compared with results from enzyme-linked immunosorbent assay (ELISA). The low RE value proves that the aptamer sensor will be a promising method for ABA detection.

Preparation of Fe3O4@PDA@Au@GO Composite as SERS Substrate and Its Application in the Enrichment and Detection for Phenanthrene

In this study, highly active Fe₃O₄@PDA@Au@GO surface-enhanced Raman spectroscopy (SERS) active substrate was synthesized for application in the enrichment and detection of trace polycyclic aromatic hydrocarbons (PAHs) in the environment. The morphology and structure were characterized by transmission electron microscopy (TEM), energy-dispersive spectroscopy (EDS), X-ray diffraction (XRD) and UV–visible absorption spectrum (UV–vis spectra). The effect of each component of Fe₃O₄@PDA@Au@GO nanocomposites on SERS was explored, and it was found that gold nanoparticles (Au NPs) are crucial to enhance the Raman signal based on the electromagnetic enhancement mechanism, and apart from enriching the PAHs through π–π interaction, graphene oxide (GO) also generates strong chemical enhancement of Raman signals, and polydopamine (PDA) can prevent Au from shedding and agglomeration. The existence of Fe3O4 aided the quick separation of substrate from the solutions, which greatly simplified the detection procedure and facilitated the reuse of the substrate. The SERS active substrate was used to detect phenanthrene in aqueous solution with a detection limit of 10⁻⁷ g/L (5.6 × 10⁻¹⁰ mol/L), which is much lower than that of ordinary Raman, it is promising for application in the enrichment and detection of trace PAHs.

Long-Lasting Luminol Chemiluminescence Emission with 1,10-Phenanthroline-2,9-dicarboxylic Acid Copper(II) Complex on Paper

As most of the known systems are flashtype, long-lasting chemiluminescence (CL) emissions are extremely needed for the application of cold light sources, accurate CL quantitative analysis, and biological mapping. In this work, the flashtype system of luminol was altered to a long lasting CL system just because of the paper substrate. The Cu(II)-based organic complex was loaded on the paper surface, which can trigger luminol-H₂O₂ to produce a long lasting CL emission for over 30 min. By using 1,10-phenanthroline-2,9-dicarboxylic acid (PDA) as the ligand, a hexacoordinated Cu(II)-based organic complex was synthesized by the simple freeze-drying method. It is interesting that the complex morphology can be controlled by adding different amounts of water in the synthesizing procedure. The complex with a certain size can be definitely trapped in the pores of the cellulose. Then, slow diffusion, which can be attributed to the long lasting CL emission, was produced. With the high catalytic activity of the complex, reactive oxygen species from H₂O₂ was generated and was responsible for the high CL intensity. By using the paper substrate, the flash-type luminol system can be easily transferred to the long-duration CL system without any extra reagent. This long-lasting emission system was used for hydrogen sulfide detection by the CL imaging method. This paper-based sensor has great potential for CL imaging in the clinical field in the future.

Lab-on-Paper Devices for Diagnosis of Human Diseases Using Urine Samples-A Review

In recent years, microfluidic lab-on-paper devices have emerged as a rapid and low-cost alternative to traditional laboratory tests. Additionally, they were widely considered as a promising solution for point-of-care testing (POCT) at home or regions that lack medical infrastructure and resources. This review describes important advances in microfluidic lab-on-paper diagnostics for human health monitoring and disease diagnosis over the past five years. The review commenced by explaining the choice of paper, fabrication methods, and detection techniques to realize microfluidic lab-on-paper devices. Then, the sample pretreatment procedure used to improve the detection performance of lab-on-paper devices was introduced. Furthermore, an in-depth review of lab-on-paper devices for disease measurement based on an analysis of urine samples was presented. The review concludes with the potential challenges that the future development of commercial microfluidic lab-on-paper platforms for human disease detection would face.

Recent advances in lab-on-paper diagnostic devices using blood samples

Lab-on-paper, or microfluidic paper-based analytical devices (μPADs), use paper as a substrate material, and are patterned with a system of microchannels, reaction zones and sensing elements to perform analysis and detection. The sample transfer in such devices is performed by capillary action. As a result, external driving forces are not required, and hence the size and cost of the device are significantly reduced. Lab-on-paper devices have thus attracted significant attention for point-of-care medical diagnostic purposes in recent years, particularly in less-developed regions of the world lacking medical resources and infrastructures. This review discusses the major advances in lab-on-paper technology for blood analysis and diagnosis in the past five years. The review focuses particularly on the many clinical applications of lab-on-paper devices, including diabetes diagnosis, acute myocardial infarction (AMI) detection, kidney function diagnosis, liver function diagnosis, cholesterol and triglyceride (TG) analysis, sickle-cell disease (SCD) and phenylketonuria (PKU) analysis, virus analysis, C-reactive protein (CRP) analysis, blood ion analysis, cancer factor analysis, and drug analysis. The review commences by introducing the basic transmission principles, fabrication methods, structural characteristics, detection techniques, and sample pretreatment process of modern lab-on-paper devices. A comprehensive review of the most recent applications of lab-on-paper devices to the diagnosis of common human diseases using blood samples is then presented. The review concludes with a brief summary of the main challenges and opportunities facing the lab-on-paper technology field in the coming years.

Sensitive and reliable identification of fentanyl citrate in urine and serum using chloride ion-treated paper-based SERS substrate

Recently, the phenomenon of fentanyls overdose leading to death is emerging in an endless stream. There is an urgent requirement to quickly identify fentanyl content in body fluids for medical and judicial purposes. With this in mind, we present a paper-based SERS substrate decorated with uniform gold nanospheres treated by chloride ion for the detection of fentanyl citrate in urine and serum. In particular, the paper-based SERS sensor was prepared by liquid/liquid self-assembly technique and chloride ion was introduced to clean and modify the substrate surface, which improved the sensitivity of the solid substrate with an enhancement factor (EF) as high as 1.64 × 10⁵. Moreover, the uniformity of each paper-based substrate and the repeatability on different batches of substrate were excellent, and there was no obvious change in the intensity response of Raman spectra within a month. As a result, the quantitative analysis of fentanyl citrate in artificial urine and rat serum were performed based on the modified paper-based substrate with the limit of detection as low as 0.59 μg/mL and 2.78 μg/mL, respectively. Both the concentrations of the two biological samples with the Raman signal intensity were linearly plotted and the recovery of the spiked samples with different concentrations was collected to verify the accuracy of the quantitative curves. All the results suggest that this work makes SERS method available for the rapid identification and quantitative analysis of illicit drug in the real biological samples.

UiO-66 metal-organic frameworks/gold nanoparticles based substrates for SERS analysis of food samples

Recently, metal-organic frameworks (MOFs) based substrates have shown great potential for the quantitative analysis of food samples by surface-enhanced Raman scattering (SERS) due to their unique properties. Herein, we developed two UiO-66 MOFs/gold nanoparticles (AuNPs) based substrates by self-assembly, including UiO-66/AuNPs suspension substrate and UiO-66(NH₂)/AuNPs/Nylon-66 flexible membrane substrate, for quantitative analysis of complex food samples by SERS. UiO-66/AuNPs suspension substrate was prepared for SERS-based determination of a carcinogenic heterocyclic amine in barbecue meat. UiO-66(NH₂)/AuNPs/Nylon-66 membrane substrate was fabricated for the simultaneous separation, enrichment, and in situ analysis of Sudan Red 7B in chilli products. The heterocyclic amine and Sudan dye in real samples could be detected and quantified with the recoveries of 82.3-110% and 84.5-114% and relative standard deviations (RSDs) of 3.1-11.0% and 1.9-5.6% (n = 3) by use of these two substrates, respectively. These two UiO-66/AuNPs based substrates combined molecular enrichment and SERS activity, achieving excellent analytical accuracy and widening SERS application in practical food safety analysis.

Self-assembly of colloidal nanoparticles into 2D arrays at water-oil interfaces: rational construction of stable SERS substrates with accessible enhancing surfaces and tailored plasmonic response

Self-assembly at water-oil interfaces has been shown to be a cheap, convenient and efficient route to obtain densely packed layers of plasmonic nanoparticles which have small interparticle distances. This creates highly plasmonically active materials that can be used to give strong SERS enhancement and whose structure means that they are well suited to creating the highly stable, reproducible and uniform substrates that are needed to allow routine and accurate quantitative SERS measurements. A variety of methods have been developed to induce nanoparticle self-assembly at water-oil interfaces, fine tune the surface chemistry and adjust the position of the nanoparticles at the interface but only some of these are compatible with eventual use in SERS, where it is important that target molecules can access the active surface unimpeded. Similarly, it is useful to transform liquid plasmonic arrays into easy-to-handle free-standing solid films but these can only be used as solid SERS substrates if the process leaves the surface nanoparticles exposed. Here, we review the progress made in these research areas and discuss how these developments may lead towards achieving rational construction of tailored SERS substrates for sensitive and quantitative SERS analysis.

Quantitative detection of dithiocarbamate pesticides by surface-enhanced Raman spectroscopy combined with an exhaustive peak-seeking method

Surface-enhanced Raman spectroscopy (SERS) based on nanosilver colloid substrates has great potential for rapid detection of pesticide residues because of its advantages of sensitivity, rapidity, simplicity, low cost, etc. However, its poor repeatability and narrow linear quantitative range limit its practical application. In this paper, a silver colloid SERS analysis method combined with an exhaustive peak-seeking method was introduced for quantitative determination of thiram and ziram. This method can establish a linear quantitative relationship in a wide range by use of an own characteristic peak of analysis as an internal standard (IS) which is found via judging the linear correlation between the intensity ratios of two SERS peaks of analytes and the concentrations. Combined with improving the preparation method of silver colloids, adding suitable activators and optimizing the detection process, a liquid detection system with good repeatability and a wide linear quantitation range was obtained. The relative standard deviation (RSD) of the strongest SERS peak is no more than 8.98%, which is better than the general case of the silver colloid SERS substrate. The ratio of I₁₃₈₄/I₁₁₄₈ has a good linear relationship with the concentration of thiram solution, and the 1148 cm^-1 characteristic peak was utilized as the IS to establish the standard curve equation for the determination of thiram concentration. The equation is I₁₃₈₄/I₁₁₄₈ = -1.7930 × lg[c_thiram (ppm)] + 6.0078 with a linear range of 10^-2 to 10² ppm (4.16 × 10^-8 to 4.16 × 10^-4 mol L^-1) and a limit of detection (LOD) of 10^-2 ppm. The peak of IS for the determination of ziram concentration is at 938 cm^-1, and the equation is I₁₃₈₄/I₉₃₈ = 4.5531 × lg[c_ziram (ppm)] + 6.4792 with a linear range of 10^-1 to 10² ppm (3.27 × 10^-7 to 3.27 × 10^-4 mol L^-1) and a LOD of 10^-4 ppm. Thiram or ziram in apple juice was successfully detected by using this liquid detection system. This analysis system effectively solves the problem of poor repeatability and a narrow linear quantification range in SERS analysis based on silver colloid substrates, and the linear quantification range meets the requirements of the national standard (GB-2763-2019).

Paper-Based Microfluidic Sensors for Onsite Environmental Detection: A Critical Review

A newly developed research topic, fabricated paper-based microfluidic sensors, was discussed in the field of low-cost environmental detection. Distinguished with the traditional dipstick or lateral-flow setups, these paper-based microfluidic sensors can serve as a tool for onsite quantitative and semi-quantitative measurements, without risks to cause environmental pollution. They have attracted increasing interest since the first easy-fabricated paper-based setup reported by Whitesides group in 2007. Most of the publications utilized paper-based sensors in clinical detection. In recent years, some groups started to use these sensors in environmental measurement, leading to precise, easy operation, low-cost, and eco-friendly methods for onsite detection. In this review, paper-based microfluidic sensors were briefly introduced, followed by literatures review and discussion for future perspectives.

Optical Screening Methods for Pesticide Residue Detection in Food Matrices: Advances and Emerging Analytical Trends

Pesticides have been extensively used in agriculture to protect crops and enhance their yields, indicating the need to monitor for their toxic residues in foodstuff. To achieve that, chromatographic methods coupled to mass spectrometry is the common analytical approach, combining low limits of detection, wide linear ranges, and high accuracy. However, these methods are also quite expensive, time-consuming, and require highly skilled personnel, indicating the need to seek for alternatives providing simple, low-cost, rapid, and on-site results. In this study, we critically review the available screening methods for pesticide residues on the basis of optical detection during the period 2016-2020. Optical biosensors are commonly miniaturized analytical platforms introducing the point-of-care (POC) era in the field. Various optical detection principles have been utilized, namely, colorimetry, fluorescence (FL), surface plasmon resonance (SPR), and surface enhanced Raman spectroscopy (SERS). Nanomaterials can significantly enhance optical detection performance and handheld platforms, for example, handheld SERS devices can revolutionize testing. The hyphenation of optical assays to smartphones is also underlined as it enables unprecedented features such as one-click results using smartphone apps or online result communication. All in all, despite being in an early stage facing several challenges, i.e., long sample preparation protocols or interphone variation results, such POC diagnostics pave a new road into the food safety field in which analysis cost will be reduced and a more intensive testing will be achieved.

SERS based sensor for mycotoxins detection: Challenges and improvements

Surface-enhanced Raman spectroscopy (SERS) has become a growing ultrasensitive analytical technique to quantify toxic molecules in foodstuffs. Monitoring the levels of chemical contaminants not only ensures food security but also offers a guideline on the production, processing, and risk analysis of consumer's health protection. The objective of this study was to point out the possible challenges associated with the detection of mycotoxins in foodstuffs. Herein, we have discussed briefly as to selectivity, accuracy, precision, robustness, ruggedness, non-specific adsorption (NSA), cross-reactivity (for both label-free and the target analyte capture approaches like the application of antibody, aptamer, molecularly imprinted polymer (MIP), linear polymer affinity agents and/or specific surface-modified nanomaterials) and their potential solution.

Multicolor nitrogen dots for rapid detection of thiram and chlorpyrifos in fruit and vegetable samples

The development of sensitive fluorescence sensors and efficient preparation of samples is a challenge in the detection of pesticides in complex samples. In this study, multicolor nitrogen dots (M-Ndots) were synthesised via microwave irradiation at 140 °C for 10 min with 5-amino-1H-tetrazole and p-phenylenediamine as precursors, which have a high fluorescence quantum yield of up to 31%. Furthermore, the M-Ndots were employed as fluorescence sensors for pesticide detection by being combined with a gas membrane separation device, to eliminate the interference from the complex sample matrix. In this process, the M-Ndots were used for sensing thiram and chlorpyrifos through their affinities to Cu²⁺ and Fe³⁺, respectively. Because thiram could decompose into volatile CS₂, its derivate was sensed using the fluorescence of M-Ndots via a complexation reaction with Cu²⁺. Chlorpyrifos, due to its volatility, can reduce the Fe³⁺ ion by inhibiting the activity of acetylcholinesterase, which produces H₂O₂ to oxidise Fe²⁺. In a real application, the time consumption for 96 samples was less than 30 min in one run of the gas membrane separation device. The recoveries for thiram and chlorpyrifos ranged from 90.0% to 115.0%, and the analytical results were validated using LC-MS/MS methods, with relative errors ranging from -7.4% to 10.1%.