Surface-enhanced Raman scattering aptasensor for ultrasensitive trace analysis of bisphenol A

Monolithic 3D structural-substrate SERS sensing platform for ultrasensitive and highly-specific analysis of trace bisphenol A

Constructing advanced substrates with excellent features is promising for sensitive surface-enhanced Raman spectroscopy (SERS) detection. Here a novel capillary monolithic 3D structural-substrate SERS platform with Au@cDNA@Ag@Cyanine 3-aptamer nanoparticles (Au@cDNA@Ag@Cy3-Apt NPs) was fabricated for rapid, highly specific profiling of ultra-trace Bisphenol A (BPA). The proposed SERS platform combined both in-capillary SERS and aptamer-affinity recognition strategies, in which the superior SERS properties of Au-Ag NPs, aptamer selectivity, and the advantages of capillary monolith were integrated. A 3D hierarchically porous network was constructed in the monolithic column, which was endowed with rich hotspots for SERS, rapid sample permeation, and better analysis efficiency than most plane-shaped SERS modes. By varying the amount of Ag+ precursor, the Ag-shell thickness on SERS was finely tuned to guarantee Cy3 label in proximity to the plasmonic surface. Based on the biorecognition of aptamer, the selective identification of BPA occurred and exhibited a significant change in SERS intensity without obvious interference. As a result, the monolithic SERS platform featured facile operation, excellent specificity, and rapid analysis (10 min, much less than the solution-based or planar substrate SERS modes). Ultra-high sensitivity and robust reproducibility for BPA analysis was achieved with a low limit of detection (LOD) at 9.12 × 10-4 ng/L. The feasibility of this SERS platform for monitoring BPA in water and milk samples was also validated. This work lights a new access to capillary monolithic SERS-sensing platform for ultrasensitive and specific analysis of BPA.

Facile Chemical Synthesis of Co-Ru-Based Heterometallic Supramolecular Polymer for Electrochemical Oxidation of Bisphenol A: Kinetics Study at the Electrode/Electrolyte Interface

Regardless of the adverse effects of Bisphenol A (BPA), its use in industry and in day-to-day life is increasing at a higher rate every year. In the present study, a simple and reliable chemical approach was used to develop an efficient BPA sensor based on a Co-Ru-based heterometallic supramolecular polymer (polyCoRu). Surface morphology and elemental analysis were examined using scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX). Furthermore, functional group analysis was accomplished by Fourier transform infrared spectroscopy (FT-IR). UV-vis spectroscopy was used to confirm the complexation in the ratio of 0.5:0.5:1 (metal 1/metal 2/ligand). Electrochemical characterization of the synthesized polyCoRu was conducted using cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) analyses. The study identified two distinct linear dynamic ranges for the detection of BPA, 0.197-2.94 and 3.5-17.72 μM. The regression equation was utilized to determine the sensitivity and limit of detection (LOD), resulting in values of 0.6 μA cm-2 μM-1 and 0.02 μM (S/N = 3), respectively. The kinetics of BPA oxidation at the polyCoRu/GCE were investigated to evaluate the heterogeneous rate constant (k), charge transfer coefficient (α), and the number of electrons transferred during the oxidation and rate-determining step. A probable electrochemical reaction mechanism has been presented for further comprehending the phenomena occurring at the electrode surface. The practical applicability of the fabricated electrode was analyzed using tap water, resulting in a high percentage of recovery ranging from 96 to 105%. Furthermore, the reproducibility and stability data demonstrated the excellent performance of polyCoRu/GCE.

Ultrasensitive Detection of Malachite Green Isothiocyanate Using Nanoporous Gold as SERS Substrate

In this article, a high-performance nanostructured substrate has been fabricated for the ultrasensitive detection of the organic pollutant, Malachite green isothiocyanate (MGITC), in aquatic systems via the Surface Enhanced Raman Spectroscopy (SERS) technique. The chemical dealloying approach has been used to synthesize a three-dimensional nanoporous gold substrate (NPG) consisting of pores and multigrained ligament structures along thickness. The formation of the framework in NPG-5h has been confirmed by SEM with an average ligament size of 65 nm at the narrower neck. Remarkable SERS performance has been achieved by utilizing the NPG-5h substrate for the detection of MGITC, showing a signal enhancement of 7.9 × 10⁹. The SERS substrate also demonstrated an impressively low-detection limit of 10^-16 M. The presence of numerous active sites, as well as plasmonic hotspots on the nanoporous surface, can be accredited to the signal amplification via the Localized Surface Plasmon Resonance (LSPR) phenomenon. As a result, SERS detection technology with the fabricated-NPG substrate not only proves to be a simple and effective approach for detecting malachite green but also provides a basis for in situ detection approach of toxic chemicals in aquatic ecosystems.

Advancements in nanomaterial-based aptasensors for the detection of emerging organic pollutants in environmental and biological samples

The combination of nanomaterials (NMs) and aptamers into aptasensors enables highly specific and sensitive detection of diverse pollutants. The great potential of aptasensors is recognized for the detection of diverse emerging organic pollutants (EOPs) in different environmental and biological matrices. In addition to high sensitivity and selectivity, NM-based aptasensors have many other advantages such as portability, miniaturization, facile use, and affordability. This work showcases the recent advances achieved in the design and fabrication of NM-based aptasensors for monitoring EOPs (e.g., hormones, phenolic contaminants, pesticides, and pharmaceuticals). On the basis of their sensing mechanisms, the covered aptasensing systems are classified as electrochemical, colorimetric, PEC, fluorescence, SERS, and ECL. Special attention has been paid to the fabrication processes, analytical achievements, and sensing mechanisms of NM-based aptasensors. Further, the practical utility of aptasensing approaches has also been assessed based on their basic performance metrics (e.g., detection limits, sensing ranges, and response times).

Flexible nano-cloth-like Ag cluster@rGO with ultrahigh SERS sensitivity for capture-optimization-detection due to effective molecule-substrate interactions

Surface-enhanced Raman scattering (SERS) is a rapid and promising detection technique for trace molecules. A central goal of research in this area is to achieve the highly sensitive detection of molecules built on a systematic understanding of enhancement mechanisms. Herein, we develop a Ag cluster@rGO composite nanostructure, which utilizes strong molecular adsorption to achieve ultrahigh SERS sensitivity. Ag clusters are prepared without additional reducing agents, leaving a low carbon footprint in the fabrication process. Finite-difference time-domain (FDTD) simulations show strong electromagnetic field enhancements generated at the edges and interstices of Ag clusters due to the specificity of their structure. Density Functional Theory (DFT) calculations show that the HOMO-LUMO energy gap value is significantly reduced when Ag cluster@rGO forms a composite system with the target molecule, which enables efficient charge transfer between the substrate and molecules, resulting in charge transfer enhancement. A detection limit of 10^-14 M using our substrate can be achieved for the environmental pollutant dye rhodamine 6G (Rh6G). The detection limits of bisphenol A (BPA) and its derivatives reach nanomolar levels with good signal stability. More importantly, we demonstrate the ability to rapidly screen BPA migration in Chinese Baijiu. Our SERS platform can be further developed for environmental pollution control and food safety.

An Overview of Aptamer-Based Sensor Platforms for the Detection of Bisphenol-A

Endocrine disruptive compounds are natural or anthropogenic environmental micropollutants that alter the function of the endocrine system ultimately damaging the metabolism. Bisphenol A (BPA) is the most common of these pollutants and it is often used in epoxy coatings and polycarbonates as a plasticizer. Therefore, monitoring BPA levels in different environments is very important and challenging. In recent years, an increasing number of BPA detection methods have been proposed. This article presents a critical review of aptamer-based electrochemical, fluorescence-based, colorimetric, and several other BPA detection platforms published in the last decade. Furthermore, a statistical evaluation has been made using principle component analysis showing analytical performance parameters do not create very different clusters. Comparisons to other BPA detection methods are also presented so that the reader has an overall literature overview.

A review of spectroscopic probes constructed from aptamer-binding gold/silver nanoparticles or their dimers in environmental pollutants' detection

The issue of environmental pollutant residues has gained wide public attention all along. Therefore, it is necessary to develop simple, rapid, economical, portable, and sensitive detection techniques, which have become the focus of research in the pollutants detection field. Spectroscopy is one of the most convenient, simple, rapid, and intuitive analytical tools that can provide accurate information, such as ultraviolet spectroscopy, fluorescence spectroscopy, Raman spectroscopy, plasmon resonance spectroscopy, etc. Gold nanoparticles, silver nanoparticles, and their dimers with unique optical properties are commonly used in the construction of spectroscopic probes. As a class of oligonucleotides that can recognize specific target molecules, aptamers also have a strong ability to recognize small-molecule pollutants. The application of aptamer-binding metal nanoparticles in biosensing detection presents significant advantages for instance high sensitivity, good selectivity, and rapid analysis. And many spectroscopic probes constructed by aptamer-binding gold nanoparticles, silver nanoparticles, or their dimers have been successfully demonstrated for detecting pollutants. This review summarizes the progress, advantages, and disadvantages of aptamer sensing techniques constructed by visual colorimetric, fluorescence, Raman, and plasmon resonance spectroscopic probes combining gold/silver nanoparticles or their dimers in the field of pollutants detection, and discusses the prospects and challenges for their future.

Direct and Sensitive Electrochemical Detection of Bisphenol A in Complex Environmental Samples Using a Simple and Convenient Nanochannel-Modified Electrode

Rapid, convenient, and sensitive detection of Bisphenol A (BPA) in complex environmental samples without the need for tedious pre-treatment is crucial for assessing potential health risks. Herein, we present an electrochemical sensing platform using a simple nanochannel-modified electrode, which enables the direct and sensitive detection of BPA in complex samples. A vertically ordered mesoporous silica-nanochannel film (VMSF) with high-density nanochannels is rapidly and stably grown on the surface of a electrochemically activated glassy carbon electrode (p-GCE) by using the electrochemically assisted self-assembly (EASA) method. The high antifouling capability of the VMSF/p-GCE sensor is proven by investigating the electrochemical behavior of BPA in the presence of model coexisting interfering molecules including amylum, protein, surfactant, and humic acid. The VMSF/p-GCE sensor can sensitively detect BPA ranged from 50 to 1.0 μM and 1.0–10.0 μM, with low detection limits (15 nM). Owing to the electrocatalytic performance and high potential resolution of p-GCE, the sensor exhibits high selectivity for BPA detection in the presence of common environmental pollutants, including bisphenol S (BPS), catechol (CC), hydroquinone (HQ), and 4-nitrophenol (4-NP). In combination with the good antifouling property of the VMSF, direct detection of BPA in environmental water samples and soil leaching solution (SLS) is also realized without separation pretreatment. The developed VMSF/p-GCE sensor demonstrated advantages of simple structure, high sensitivity, good antifouling performance, and great potential in direct electroanalysis of endocrine-disrupting compounds in complex samples.

Aptamer-Adjusted Carbon Dot Catalysis-Silver Nanosol SERS Spectrometry for Bisphenol A Detection

Carbon dots (CDs) can be prepared from various organic (abundant) compounds that are rich in surfaces with –OH, –COOH, and –NH₂ groups. Therefore, CDs exhibit good biocompatibility and electron transfer ability, allowing flexible surface modification and accelerated electron transfer during catalysis. Herein, CDs were prepared using a hydrothermal method with fructose, saccharose, and citric acid as C sources and urea as an N dopant. The as-prepared CDs were used to catalyze AgNO₃–trisodium citrate (TSC) to produce Ag nanoparticles (AgNPs). The surface-enhanced Raman scattering (SERS) intensity increased with the increasing CDs concentration with Victoria blue B (VBB) as a signal molecule. The CDs exhibited a strong catalytic activity, with the highest activity shown by fructose-based CDs. After N doping, catalytic performance improved; with the passivation of a wrapped aptamer, the electron transfer was effectively disrupted (retarded). This resulted in the inhibition of the reaction and a decrease in the SERS intensity. When bisphenol A (BPA) was added, it specifically bound to the aptamer and CDs were released, recovering catalytical activity. The SERS intensity increased with BPA over the concentration range of 0.33–66.67 nmol/L. Thus, the aptamer-adjusted nanocatalytic SERS method can be applied for BPA detection.

Aptamer-Based Biosensors for the Analytical Determination of Bisphenol A in Foodstuffs

Bisphenol A (BPA) is a synthetic compound utilized to manufacture plastics for Food Contact Materials (FCMs) or resins for the inside of food containers. Since it was recognized as an Endocrine-Disrupting Chemical (EDC), its implications in pathologies, such as cancer, obesity, diabetes, immune system alterations, and developmental and mental disorders, have been widely documented. Diet is considered the main source of exposure for humans to BPA. Consequently, continuous monitoring of the levels of BPA in foods is necessary to assess the risk associated with its consumption in one’s diet. So far, many reviews have been published on biosensors and aptamer-based biosensors, but none of them focus on their applications in their analyses of bisphenols in food matrices. With this review, the authors aim to fill this gap and to take a snapshot of the current state-of-the-art research on aptasensors designed to detect BPA in food matrices. Given that a new TDI value has recently been proposed by the EFSA (0.04 ng/kg), the search for new sensitive tools for the quantitative analysis of BPA is more topical and urgent than ever. From this perspective, aptasensors prove to be a good alternative to traditional analytical techniques for determining BPA levels in food.

The hazardous threat of Bisphenol A: Toxicity, detection and remediation

Bisphenol A (or BPA) is a toxic endocrine disrupting chemical that is released into the environment through modern manufacturing practices. BPA can disrupt the production, function and activity of endogenous hormones causing irregularity in the hypothalamus-pituitary-gonadal glands and also the pituitary-adrenal function. BPA has immuno-suppression activity and can downregulate T cells and antioxidant genes. The genotoxicity and cytotoxicity of BPA is paramount and therefore, there is an immediate need to properly detect and remediate its influence. In this review, we discuss the toxic effects of BPA on different metabolic systems in the human body, followed by its mechanism of action. Various novel detection techniques (LC-MS, GC-MS, capillary electrophoresis, immunoassay and sensors) involving a pretreatment step (liquid-liquid microextraction and molecularly imprinted solid-phase extraction) have also been detailed. Mechanisms of various remediation strategies, including biodegradation using native enzymes, membrane separation processes, photocatalytic oxidation, use of nanosorbents and thermal degradation has been detailed. An overview of the global regulations pertaining to BPA has been presented. More investigations are required on the efficiency of integrated remediation technologies rather than standalone methods for BPA removal. The effect of processing operations on BPA in food matrices is also warranted to restrict its transport into food products.

Current progress in aptamer-based sensing tools for ultra-low level monitoring of Alzheimer's disease biomarkers

Alzheimer's disease (AD) as common late-life dementia is pathologically associated with the irreversible and progressive disorder, misfolding, deposition, and accumulation of the brain proteins. Especially, the formation of fibrous amyloid plaques by aggregation of amyloid-β peptides is the pathological cause of this neurologic disorder disease. Besides, tau protein isoforms destabilize the microtubule filaments through post-translational modifications and induce nerve cells' death. Amyloid-β peptides and tau proteins are considered as the critical symptom and reliable molecular biomarkers for the early diagnosis of AD. AD is characterized by impaired thinking proficiencies, cognitive decline, memory loss, and behavioral disability. Since there is no efficacious therapy for AD at present, the development of precise sensing tools for the early diagnosis of this disease is essential and crucial. Aptamer-based biosensors (aptasensors) have acquired utmost importance in the field of AD healthcare, due to excellent sensitivity and specificity, ease-of-use, cost-effectiveness, portability, and rapid assay time. Here, we highlight the recent developments and novel perspectives in the field of aptasensor design to quantitatively monitor the AD biomarkers. Finally, some results are represented to achieve a promising viewpoint for introducing the novel aptasensor test kits in the future.

Recent Advances Towards Point-Of-Care Applications of Surface-Enhanced Raman Scattering Sensing

The ability to accurately diagnose at the point of care is crucial in many pathologies. However, current standard diagnostic practices can only be performed in specialized health or laboratory settings. To move diagnostic methods from a specialized lab to the point of care many alternate methods have been developed and proposed. Among them surface-enhanced Raman scattering (SERS) sensing offers advantageous features, such as simultaneous detection of multiple biotargets and increased accuracy. Many groups have been working towards the translation of SERS sensing methods from the lab to the point of need. In this mini review, we discuss interesting and recent developments in this effort, focusing on how different sensing mechanism can be used in point-of-care testing applications of SERS.

Recent Advances in Hollow Gold Nanostructures for Biomedical Applications

The localized surface plasmon resonance of metallic nanoparticles has attracted much attention owing to its unique characteristics, including the enhancement of signals in sensors and photothermal effects. In particular, hollow gold nanostructures are highly promising for practical applications, with significant advantages being found in their material properties and structures: 1) the interaction between the outer surface plasmon mode and inner cavity mode leads to a greater resonance, allowing it to absorb near-infrared light, which can readily penetrate tissue; 2) it has anti-corrosiveness and good biocompatibility, which makes it suitable for biomedical applications; 3) it shows a reduced net density and large surface area, allowing the possibility of nanocarriers for drug delivery. In this review, we present information on the classification, characteristics, and synthetic methods of hollow gold nanostructures; discuss the recent advances in hollow gold nanostructures in biomedical applications, including biosensing, bioimaging, photothermal therapy, and drug delivery; and report on the existing challenges and prospects for hollow gold nanostructures.

SERS imaging-based aptasensor for ultrasensitive and reproducible detection of influenza virus A

Surface-enhanced Raman scattering (SERS)-based aptasensors display high sensitivity for influenza A/H1N1 virus detection but improved signal reproducibility is required. Therefore, in this study, we fabricated a three-dimensional (3D) nano-popcorn plasmonic substrate using the surface energy difference between a perfluorodecanethiol (PFDT) spacer and the Au layer. This energy difference led to Au nanoparticle self-assembly; neighboring nanoparticles then created multiple hotspots on the substrate. The localized surface plasmon effects at the hot spots dramatically enhanced the incident field. Quantitative evaluation of A/H1N1 virus was achieved using the decrease of Raman peak intensity resulting from the release of Cy3-labeled aptamer DNAs from nano-popcorn substrate surfaces via the interaction between the aptamer DNA and A/H1N1 virus. The use of a Raman imaging technique involving the fast mapping of all pixel points enabled the reproducible quantification of A/H1N1 virus on nano-popcorn substrates. Average ensemble effects obtained by averaging all randomly distributed hot spots mapped on the substrate made it possible to reliably quantify target viruses. The SERS-based imaging aptasensor platform proposed in this work overcomes the issues inherent in conventional approaches (the time-consuming and labor-intensiveness of RT-PCR and low sensitivity and quantitative analysis limits of lateral flow assay kits). Our SERS-based assay for detecting A/H1N1 virus had an estimated limit of detection of 97 PFU mL-1 (approximately three orders of magnitude more sensitive than that determined by the enzyme-linked immunosorbent assay) and the approximate assay time was estimated to be 20 min. Thus, this approach provides an ultrasensitive, reliable platform for detecting viral pathogens.

A simple fluorescence anisotropy assay for detection of bisphenol A using fluorescently labeled aptamer

Bisphenol A (BPA) is one of the environmental endocrine disruptors (EDCs), and BPA contamination in environment can cause high risks to human health. Rapid determination of BPA on sites is in high demand in environmental analysis. Taking advantage of aptamers as affinity ligands and fluorescence anisotropy (FA) analysis, we developed a simple and rapid FA assay for BPA by employing a single tetramethylrhodamine (TMR) labeled short 35-mer DNA aptamer against BPA. The assay is based on the BPA-binding induced conformation change of TMR-labeled aptamer and alteration of interaction between TMR and guanine bases, resulting in change of FA signals. We screened the FA change of aptamer probes having TMR label on a specific site of the aptamer upon BPA addition. The aptamer with a TMR label on the 22nd T base showed large FA-decreasing response to BPA and maintained good binding affinity to BPA. By using this TMR-labeled aptamer, we achieved FA detection of BPA with a detection limit of 0.5 μmol/L under the optimized conditions. This assay was selective towards BPA and enabled the detection of BPA spiked in tap water sample, showing the potential applications on water samples.

Development of a new bisphenol A electrochemical sensor based on a cadmium(ii) porphyrin modified carbon paste electrode

In this study, the (5,10,15,20-tetrakis[(4-methoxyphenyl)]porphyrinato)cadmium(ii) complex ([Cd(TMPP)]) was successfully used as a modifier in a carbon paste electrode (CPE) and exploited for bisphenol A (BPA) detection. Analytical performance revealed two linear ranges from 0.0015–15 μM and 0.015–1.5 mM with a detection limit of 13.5 pM. The proposed method was implemented in water samples, which resulted in quantitative signals over the range 6.5–1000 μM with recoveries between 92.6 and 107.7% for tap water and between 96.6 to 106.0% for mineral water.

In this study, the (5,10,15,20-tetrakis[(4-methoxyphenyl)]porphyrinato)cadmium(ii) complex ([Cd(TMPP)]) was successfully used as a modifier in a carbon paste electrode (CPE) and exploited for bisphenol A (BPA) detection.

Ultrasensitive SERS detection of rhodamine 6G and p-nitrophenol based on electrochemically roughened nano-Au film

Quantitative analysis of organic pollutants in environmental water is an important issue for ecological environment and human health. In this paper, the quantitative analysis of rhodamine 6G (R6G) and p-nitrophenol (PNP) is performed by the surface enhanced Raman scattering (SERS) technology. The enhancement of Raman signals is achieved on the surface of an electrochemically roughened nano-Au film. The SERS performance depends on the microstructure of roughened nano-Au films, which is affected by the thickness of Au films and electrochemical roughening parameters. The structure-dependence of SERS performance is validated by finite element simulation of local electromagnetic field distribution. An obvious SERS effect of R6G with an enhancement factor of 10⁸ is obtained on the roughened nano-Au film. A sensitive SERS detection of R6G with a linear range of 10^-9-10^-5 M and a detection limit of 10^-11 M is realized. Moreover, a wide linear range of 10^-9-10^-3 M is obtained for the detection of PNP. The roughened nano-Au film is an effective substrate for the SERS detection of organic pollutants with high reproducibility and good stability. Therefore, the electrochemical technology in this study is expected to be a very promising method for the fabrication of high-performance SERS substrate.

An electrochemical aptasensor based on eATRP amplification for the detection of bisphenol A

Herein, a novel aptasensor was constructed for ultrasensitive detection of bisphenol A (BPA). In this method, an electrochemically mediated atom transfer radical polymerization (eATRP) signal amplification strategy was applied to BPA detection for the first time. The 5'-end modified sulfhydryl group and the 3'-end modified azide group hairpin DNA were immobilized on a gold electrode through an Au-S bond. The double-stranded DNA was formed by the hybridization of an aptamer and a single-stranded DNA partially paired with the hairpin DNA. In the presence of BPA, the aptamer combined with BPA and the single-stranded DNA was released to open the hairpin structure, making the azide groups at the 3' end exposed. Subsequently the initiator of eATRP was introduced into hairpin DNA through click chemistry reaction and eATRP was conducted for the polymerization of the electroactive probe ferrocene methyl methacrylate (FMMA). As a result, the ultrasensitive detection of BPA was realized, and the detection limit of this aptasensor was as low as 59 aM and a good selectivity was obtained in the presence of 100-fold structural analogs. The application of this aptasensor was evaluated by detecting BPA in pure water samples, and recoveries were in the range of 95.23-98.40%, holding promising applications in biological analysis.

Using Ca-doped carbon dots as catalyst to amplify signal to determine ultratrace thrombin by free-label aptamer-SERS method

The highly catalytic Ca-doped carbon dots (CD_Ca) were prepared by microwave procedure, that exhibit strong catalytic effect on HAuCl₄-glucose (GLC) reaction to form gold nanoparticles (AuNPs) with high SERS activity, using Victoria blue B (VBB) as a molecular probe. The SERS intensity at 1615 cm^-1 increased linearly with CD_Ca increasing, due to formation of more AuNPs nanosol substrate as indicator. When thrombin aptamer (Apt) was added in this system, Apt adsorbed on the CD_Ca surface to inhibit theirs catalytic activity, and the SERS intensity decreased. However, when thrombin (TB) was present, it can bind to Apt to form stable G-duplex of Apt-TB and free CD_Ca catalyst in the system, and the SERS signal increased linearly. Thus a free-label Apt-SERS quantitative analysis method was developed for ultratrace TB, with a linear range of 0.0058-0.115 nmol/L and a detection limit of 0.0018 nmol/L TB.

Ultrasensitive analysis of kanamycin residue in milk by SERS-based aptasensor

An ultrasensitive method for the kanamycin (KANA) detection in milk sample using surface-enhanced Raman spectroscopy-based aptasensor was employed in the current study. Double strand DNA binding bimetallic gold@silver nanoparticles were developed as a sensing platform. Probe DNAs were first embedded on the surface of gold nanoparticles by the end-modified thiol, and after silver shell encapsulating, KANA aptamer DNAs with the Raman reporter Cy3 were then hybridized with probe DNAs by complementary base pairing. Results showed that with increase in the KANA concentration, the Raman intensity of Cy3 decreased. Besides achieving selectivity, an ultralow detection limit of 0.90 pg/mL, a broad linear relationship ranging from 10 μg/mL to 100 ng/mL in aqueous reagent and satisfactory recoveries of 90.4-112% in liquid whole milk were obtained. The result of actual sample proved that this aptasensor was promising in trace determination of KANA residue.

Monitoring 2,3',5,5'-tetrachlorobiphenyl with a rapid and sensitive environmental aptamer sensor

Polychlorinated biphenyl (PCB) detection in the environment is significant for both environmental protection and human health. Herein, a highly sensitive aptamer sensor has been established by employing a 2,3',5,5'-tetrachlorobiphenyl (PCB72) targeting aptamer as a highly specific recognition element and a gold/silver (Au@Ag) nanocomposite as the surface-enhanced Raman spectroscopy (SERS) substrate for detecting environmental PCB72. The Au@Ag nanoparticles (NPs) exhibit a strong SERS enhancement and provide an efficient substrate for immobilizing the PCB72 aptamer and Raman signal labelled molecule, 4-mercaptobenzoic acid (4-MBA). The targeted PCB72 could competitively bind with the PCB72 aptamer, resulting in a few aptamers sticking to the Au@Ag NPs and the "hot spot" strengthening effect of the substrate. Under optimal conditions, this aptamer sensor exhibits great performance with high sensitivity, excellent selectivity and stability for the monitoring of PCB72, which shows an excellent linear correlation ranging from 1 to 1000 pg mL-1 with a limit of detection of 0.3 pg mL-1. Furthermore, this aptamer assay exhibits high specificity and selectivity for PCB72 with the detection error of less than 0.27 for other PCBs and 0.21 for other interfering species, even if the coexisting interferents are 100-fold concentration in the system. Additionally, the recognition mechanism of the binding of aptamers to PCB72 is analyzed via UV-vis spectroscopy and molecular docking simulations, which suggest that PCB72 could insert into the aptamers. Furthermore, this method is successfully utilized for PCB72 detection in real water samples with a simple pre-treatment. In general, this work provides a new and effective method using an environmental aptamer sensor for rapid and sensitive PCB72 detection.

Double strand DNA functionalized Au@Ag Nps for ultrasensitive detection of 17β-estradiol using surface-enhanced raman spectroscopy

A detection method for 17β-estradiol (E2) using surface-enhanced Raman scattering (SERS)-based aptamer sensor was presented. Raman reporter molecule Cy3 labeled E2-aptamer and DNA functionalized gold-silver core-shell nanoparticles (Au@Ag CS NPs) offered SERS with high sensitivity and selectivity. Based on the fabricated double strand DNA-immobilized gold-silver core-shell nanoparticles (Au@Ag NPs), SERS signal intensity of Raman reporter changed with the number of Cy3-labeled aptamer attached to the core-shell nanoparticles due to the strong binding affinity between the aptamers and E2 with different concentrations. A wide linear range from 1.0 × 10^-13 to 1.0 × 10^-9 was obtained for the detection of E2, with a low detection limit of 2.75 fM. This proposed method showed highly sensitive and selective for detecting E2, and could be used to determine E2 in actual samples.

Critical Review: DNA Aptasensors, Are They Ready for Monitoring Organic Pollutants in Natural and Treated Water Sources?

There is a growing need to monitor anthropogenic organic contaminants detected in water sources. DNA aptamers are synthetic single-stranded oligonucleotides, selected to bind to target contaminants with favorable selectivity and sensitivity. These aptamers can be functionalized and are used with a variety of sensing platforms to develop sensors, or aptasensors. In this critical review, we (1) identify the state-of-the-art in DNA aptamer selection, (2) evaluate target and aptamer properties that make for sensitive and selective binding and sensing, (3) determine strengths and weaknesses of alternative sensing platforms, and (4) assess the potential for aptasensors to quantify environmentally relevant concentrations of organic contaminants in water. Among a suite of target and aptamer properties, binding affinity is either directly (e.g., organic carbon partition coefficient) or inversely (e.g., polar surface area) correlated to properties that indicate greater target hydrophobicity results in the strongest binding aptamers, and binding affinity is correlated to aptasensor limits of detection. Electrochemical-based aptasensors show the greatest sensitivity, which is similar to ELISA-based methods. Only a handful of aptasensors can detect organic pollutants at environmentally relevant concentrations, and interference from structurally similar analogs commonly present in natural waters is a yet-to-be overcome challenge. These findings lead to recommendations to improve aptasensor performance.

Dual enzyme-free amplification strategy for ultra-sensitive fluorescent detection of bisphenol A in water

An ultra-sensitive strategy for bisphenol A (BPA) detection based on dual enzyme-free strategies: hybridization chain reaction (HCR) and enzyme-strand recycling reaction has been developed. The BPA aptamer can form hairpins structure by the partly self-complementary sequence. In the presence of BPA, the released BPA aptamer sequence can trigger the HCR between two hairpins to from a long nicked double-helix DNA. The tails of hairpins on the duplex DNA were closely enough to hybridize with molecular beacon (MB) on the gold nanoparticles (AuNPs) to circularly cleave the loop of MB, leading to a "turn-on" fluorescent signal. This method exhibited high sensitivity for BPA detection in a linear rang from 0.2 to 1000 pM with 0.05 pM of limit of detection. Moreover, it was successfully used for BPA detection in real water samples. Importantly, this method was simple without complex enzymatic procedure and high cost, showing a promising future for on-site detection of BPA in practical application.

Internal standard-based SERS aptasensor for ultrasensitive quantitative detection of Ag+ ion

A ratiometric surface-enhanced Raman scattering (SERS) aptasensor based on internal standard (IS) methods was proposed for the ultrasensitive and reproducible quantitative detection of silver ion (Ag⁺) with Au@Ag core-shell nanoparticle (Au@Ag NP) substrate. In principle, the thiolated 5'-Rox C-containing labeled aptamer probe (Rox-aptamer) is firstly immobilized on the SERS substrate surface and then hybridizes with the complementary DNA (cDNA) to form a rigid double-stranded DNA (dsDNA), in which the Rox Raman labels is used to produce the Raman signal. Furthermore, the pyridine is employed as an IS element to provide the ratiometric determination of target. In the presence of Ag⁺, the Rox-aptamer is turned into the cytosine (C)-Ag⁺-C mediated hairpin structure, which remarkably reduces the distance between the Rox labels and the Au@Ag NP surface responsible for a measurable 'turn-on' signal change of Rox. This IS-based ratiometric SERS aptasensor exhibits a limit of detection of 50 pM for Ag⁺ with a linear detection range from 0.1 to 100 nM and the shortcoming of irreproducibility of SERS signal could be overcome. The proposed method provides a simple, robust, and rapid approach for the sensitive and reproducible quantitative detection of Ag⁺, and it could also be used for the detection of other metal ions which exhibits specific interactions with natural or synthetic bases.

Aminolated and Thiolated PEG-Covered Gold Nanoparticles with High Stability and Antiaggregation for Lateral Flow Detection of Bisphenol A

The few lateral flow assays (LFAs) established for detecting the endocrine disrupting chemical bisphenol A (BPA) have employed citrate-stabilized gold nanoparticles (GNPs), which have inevitable limitations and instability issues. To address these limitations, a more stable and more sensitive biosensor is developed by designing strategies for modifying the surfaces of GNPs with polyethylene glycol and then testing their effectiveness and sensitivity toward BPA in an LFA. Without the application of any enhancement strategy, this modified BPA LFA can achieve a naked-eye limit of detection (LOD) of 0.8 ng mL^-1 , which is 12.5 times better than the LOD of regular BPA LFAs, and a quantitative LOD of 0.472 ng mL^-1 . This modified LFA has the potential to be applied to the detection of various antigens.

Practical Application of Aptamer-Based Biosensors in Detection of Low Molecular Weight Pollutants in Water Sources

Water pollution has become one of the leading causes of human health problems. Low molecular weight pollutants, even at trace concentrations in water sources, have aroused global attention due to their toxicity after long-time exposure. There is an increased demand for appropriate methods to detect these pollutants in aquatic systems. Aptamers, single-stranded DNA or RNA, have high affinity and specificity to each of their target molecule, similar to antigen-antibody interaction. Aptamers can be selected using a method called Systematic Evolution of Ligands by EXponential enrichment (SELEX). Recent years we have witnessed great progress in developing aptamer selection and aptamer-based sensors for low molecular weight pollutants in water sources, such as tap water, seawater, lake water, river water, as well as wastewater and its effluents. This review provides an overview of aptamer-based methods as a novel approach for detecting low molecular weight pollutants in water sources.

Plasmonic DNA hotspots made from tungsten disulfide nanosheets and gold nanoparticles for ultrasensitive aptamer-based SERS detection of myoglobin

A nanohybrid mediated SERS substrate was prepared by in-situ synthesis and assembly of gold nanoparticles (AuNPs) on exfoliated nanosheets of tungsten disulfide (WS₂) to form plasmonic hotspots. The nanohybrid surface was functionalized with specific aptamers which imparted high selectivity for the cardiac marker myoglobin (Mb). The fabricated aptasensor was read by SERS using a 532 nm laser and demonstrated significant signal enhancement, and this allowed Mb to be determined in the 10 f. mL^-1 to 0.1 μg mL^-1 concentration range. The study presents an approach to synergistically exploit the unique chemical and electromagnetic properties of both WS₂ and AuNPs for many-fold enhancement of SERS signals. Graphical abstract Schematic presentation of a nanohybrid-mediated SERS substrate prepared by in-situ assembly of gold nanoparticles (AuNPs) reduced on exfoliated nanosheets of tungsten disulfide (WS₂) to form plasmonic hot spots. Specific aptamers immobilized on the SERS surface impart high sensitivity and selectivity for the cardiac marker myoglobin (Mb).

Surface enhanced Raman scattering (SERS) based biomicrofluidics systems for trace protein analysis

In recent years, Surface Enhanced Raman Scattering (SERS) has been widely applied to many different areas, including chemical analysis, biomolecule detection, bioagent diagnostics, DNA sequence, and environmental monitor, due to its capabilities of unlabeled fingerprint identification, high sensitivity, and rapid detection. In biomicrofluidic systems, it is also very powerful to integrate SERS based devices with specified micro-fluid flow fields to further focusing/enhancing/multiplexing SERS signals through molecule registration, concentration/accumulation, and allocation. In this review, after a brief introduction of the mechanism of SERS detection on proteins, we will first focus on the effectiveness of different nanostructures for SERS enhancement and light-to-heat conversion in trace protein analysis. Various protein molecule accumulation schemes by either (bio-)chemical or physical ways, such as immuno, electrochemical, Tip-enhanced Raman spectroscopy, and magnetic, will then be reviewed for further SERS signal amplification. The analytical and repeatability/stability issues of SERS detection on proteins will also be brought up for possible solutions. Then, the comparison about various ways employing microfluidic systems to register, concentrate, and enhance the signals of SERS and reduce the background noise by active or passive means to manipulate SERS nanostructures and protein molecules will be elaborated. Finally, we will carry on the discussion on the challenges and opportunities by introducing SERS into biomicrofluidic systems and their potential solutions.

A magnetite/PMAA nanospheres-targeting SERS aptasensor for tetracycline sensing using mercapto molecules embedded core/shell nanoparticles for signal amplification

Surface-enhanced Raman scattering (SERS) biosensors have promising potential in the field of antibiotics detection because of their ultrahigh detection sensitivity. This paper reports a rapid and sensitive SERS-based magnetic nanospheres-targeting strategy for sensing tetracycline (TTC) using aptamer-conjugated magnetite colloid nanocrystal clusters (MCNCs)-polymethacrylic acid (PMAA) magnetic nanospheres (MNs) as the recognition and the Au/PATP/SiO₂ (APS) as the labels. Initially, MNs were fabricated and conjugated with the aptamers through condensation reaction. MNs possessed high saturation magnetization (Ms) value of 71.5emu/g and excellent biocompatibility, which facilitated the rapid and easy magnetic separation. Then, complementary DNA (cDNA) were loaded on the APS nanocarrier to produce a large amplification factor of Raman signals. The MNs-targeting aptasensor was thus fabricated by immobilizing the APS to the MNs' surfaces via the hybrid reaction between cDNA and aptamers. Sequel, TTC bound successfully to the aptamer upon its addition with the subsequent release of some cDNA-APS into the bulk solution. Under magnet attraction, the nanospheres were deposited together. Consequently, a display of strong SERS signals by supernatants of the resulting mixtures with increasing TTC concentrations was observed. The proposed aptasensor showed excellent performances for TTC detection along with wide linear range of 0.001-100ng/mL, low detection limit 0.001ng/mL, high sensitivity, and good selectivity to the general coexisted interferences.