Ultrasensitive Detection of Hepatotoxic Microcystin Production from Cyanobacteria Using Surface-Enhanced Raman Scattering Immunosensor

Assessing fresh water acute toxicity with Surface-Enhanced Raman Scattering (SERS)

It's well known that the toxicity of chemicals in the environment depends not only their concentrations, but more importantly, their bio-availability. Thus, the acute toxicity test of environmental water samples is of great importance in water quality evaluation. In this work, water acute toxicity was determined via SERS approach for the first time based on the reaction between Escherichia coli (E. coli) and p-benzoquinone (BQ). The E. coli was used as the subject of toxicity assay. Under normal conditions, the BQ molecules can be transformed into Hydroquinone (HQ) by the E. coli bacteria; subsequently, the BQ will continue to react with the resulting HQ to form Quinone hydroquinone (QHQ). This process could be impaired in the presence of many toxic chemicals. Bromide modified Ag NPs was then introduced for the highly sensitive SERS detection of the product (HQ and QHQ). Several key factors that may affect water acute toxicity evaluation have been explored, which include the initial BQ and E. coli concentration, the incubation time with BQ, and the sodium chloride concentration. Later, the established system was applied for the toxicity evaluation of Cu2+. It was found that the IC50 value of Cu2+ was 0.94 mg/L, which is superior compared with literature report. This study provides a promising SERS method for assessing acute toxicity in water bodies with high sensitivity and short detection time.

Analysis of the immune function of Caspase-3 in Cristaria plicata

Caspase-3 is generally considered to be the most important terminal shear enzyme in the process of apoptosis, as well as an important part of cytotoxic T lymphocytes (CTL) killing mechanism, which is confirmed to play an important role in vertebrate cell apoptosis and immune system, and is poorly reported in invertebrates. In this paper, we used bioinformatics to perform amino acid multiple sequence alignment and protein structural domain analysis, and constructed a phylogenetic tree to identify the full-length cDNA of the cloned caspase-3 of Cristaria plicata (Named CpCaspase-3). The expression of caspase-1, caspase-7, caspase-8, and caspase-9 was found to be down-regulated by double-stranded RNA interference of CpCaspase-3 in C. plicata. Some degree of disruption of the caspase signaling pathway occurs. The expression of CpCaspase-3 was affected after injection of Lipopolysaccharide (LPS), Peptidoglycan (PGN), polyinosinic-polycytidylic acid (poly(I:C)), and Aeromonas hydrophila. These results were suggested that CpCaspase-3 was involved in the immune response of C. plicata. The wound recovery process of C. plicata was simulated and CpCaspase-3 was found to promote wound recovery. An autophagy inhibition and autophagy activation model of mussels was constructed, where apoptosis and autophagy undergo crosstalk, and inhibition of autophagy induces the onset of apoptosis, and similarly autophagy activation inhibits the process of apoptosis instead. In addition, a recombinant CpCaspase-3-pEGFP-C1 plasmid was constructed for subcellular localization experiments and found that CpCaspase-3 was distributed in both the nucleus and the cytoplasm. This paper aims to unveil the immune mechanism of C. plicata and provide a theoretical basis for the healthy culture of shellfish.

Raman spectroscopy: Principles and recent applications in food safety

Food contaminant is a significant issue because of the adverse effects on human health and economy. Traditional detection methods such as liquid chromatography-mass spectroscopy for detecting food contaminants are expensive and time-consuming, and require highly-trained personnel and complicated sample pretreatment. Raman spectroscopy is an advanced analytical technique in a manner of non-destructive, rapid, cost-effective, and ultrasensitive sensing various hazards in agri-foods. In this chapter, we summarized the principle of Raman spectroscopy and surface enhanced Raman spectroscopy, the methods to process Raman spectra, the recent applications of Raman/SERS (surface-enhanced Raman spectroscopy) in detecting chemical contaminants (e.g., pesticides, antibiotics, mycotoxins, heavy metals, and food adulterants) and microbiological hazards (e.g., Salmonella, Campylobacter, Shiga toxigenic E. coli, Listeria, and Staphylococcus aureus) in foods.

Label-free MIP-SERS biosensor for sensitive detection of colorectal cancer biomarker

Early diagnosis of colorectal cancer can significantly improve the overall survival rate of patients, thus selective and sensitive detection of biomarkers in serum samples is vital for early detection and dynamic monitoring of cancer. Nucleoside diphosphate kinase NM23-H2 (NDKB) is an important biomarker and therapeutic target for the diagnosis of colorectal cancer (CRC). Here, a label-free and ultrasensitive biosensor for NDKB protein markers is presented for the first time, combining the characteristic capture selectivity of molecularly imprinted polymers (MIPs) and the ultrasensitivity of surface-enhanced Raman Spectroscopy (SERS) technique. The imprinted cavity serves as the only channel for Raman reporter to approach the SERS substrate, providing highly complementary non-covalent binding sites that selectively capture the target protein based on ionic, hydrogen bonding or hydrophobic interactions. Specific recognition of the NDKB protein will perfectly fill the imprinted cavity, which makes it difficult for the Raman reporter to get close to the SERS substrate, and the Raman signal decreases significantly, while the proteins of other structural sizes can not match the imprinted cavity. Through the change of the Raman signal, the proposed biosensor can realize the ultra-sensitive detection of NDKB, and the limit of detection (LOD) is 0.82 pg/mL. Compared with the traditional immunoassay technology, this combined approach with the advantages of low cost, fast response, high sensitivity and selectivity, provides clinical application potential for the early diagnosis of CRC.

SERS of cylindrospermopsin cyanotoxin: Prospects for quantitative analysis in solution and in fish tissue

Cylindrospermopsin (CYN), a cyanotoxin occurring in environmental waters as a cyanobacteria metabolite, has recently raised increased interest both in the scientific community and the environmental, food control and health care bodies due to the incidence of poisoning reports and the lack of prompt, effective detection and monitoring techniques. Here we report comprehensive Raman and SERS spectroscopy data on CYN cyanotoxin and provide a detailed characterization of the vibrational Raman signal based on DFT calculation as well as the adsorption properties with respect to the silver nanoparticles surface. Quantitative SERS analysis was achieved for concentrations range from 0.218 nM to 2.18 µM in aqueous solution. We further investigated the SERS discrimination of artificially intoxicated fish tissue from normal one, using linear discriminant analysis. Significant changes in SERS signal of toxic tissue compared to normal one allowed clear and fast differentiation of toxic tissue with 100% specificity/sensitivity. The cross-validation procedure provided 100% clear separation based on the SERS data. The results open reliable perspectives for SERS monitoring the environmental water bodies.

Detection and Characterization of Nodularin by Using Label-Free Surface-Enhanced Spectroscopic Techniques

Nodularin (NOD) is a potent toxin produced by Nodularia spumigena cyanobacteria. Usually, NOD co-exists with other microcystins in environmental waters, a class of cyanotoxins secreted by certain cyanobacteria species, which makes identification difficult in the case of mixed toxins. Herein we report a complete theoretical DFT-vibrational Raman characterization of NOD along with the experimental drop-coating deposition Raman (DCDR) technique. In addition, we used the vibrational characterization to probe SERS analysis of NOD using colloidal silver nanoparticles (AgNPs), commercial nanopatterned substrates with periodic inverted pyramids (Klarite^TM substrate), hydrophobic Tienta^® SpecTrim^TM slides, and in-house fabricated periodic nanotrenches by nanoimprint lithography (NIL). The 532 nm excitation source provided more well-defined bands even at LOD levels, as well as the best performance in terms of SERS intensity. This was reflected by the results obtained with the Klarite^TM substrate and the silver-based colloidal system, which were the most promising detection approaches, providing the lowest limits of detection. A detection limit of 8.4 × 10^-8 M was achieved for NOD in solution by using AgNPs. Theoretical computation of the complex vibrational modes of NOD was used for the first time to unambiguously assign all the specific vibrational Raman bands.

Detection of Alpha-Fetoprotein Using Aptamer-Based Sensors

Alpha-fetoprotein (AFP) is widely-known as the most commonly used protein biomarker for liver cancer diagnosis at the early stage. Therefore, developing the highly sensitive and reliable method of AFP detection is of essential demand for practical applications. Herein, two types of aptamer-based AFP detection methods, i.e., optical and electrochemical biosensors, are reviewed in detail. The optical biosensors include Raman spectroscopy, dual-polarization interferometry, resonance light-scattering, fluorescence, and chemiluminescence. The electrochemical biosensors include cyclic voltammetry, electrochemical impedance spectroscopy, and giant magnetic impedance. Looking into the future, methods for AFP detection that are high sensitivity, long-term stability, low cost, and operation convenience will continue to be developed.

Recent advances in nanomaterials-based optical and electrochemical aptasensors for detection of cyanotoxins

The existence of cyanotoxins poses serious threats to human health, it is highly desirable to develop specific and sensitive methods for rapid detection of cyanotoxins in food and water. Due to the distinct advantages of aptamer including high specificity, good stability and easy preparation, various aptamer-based sensors (aptasensors) have been proposed to promote the detection of cyanotoxins. In this review, we summarize recent advance in optical and electrochemical aptasensors for cyanotoxins sensing by integrating with versatile nanomaterials or innovative sensing strategies, such as colorimetric aptasensors, fluorescent aptasensors, surface enhancement Raman spectroscopy-based aptasensors, voltammetric aptasensors, electrochemical impedance spectroscopy-based aptasensors and photoelectrochemical aptasensors. We highlight the accomplishments and advancements of aptasensors with improved performance. Furthermore, the current challenges and future prospects in cyanotoxins detection are discussed from our perspectives, which we hope to provide more ideas for future researchers.

A novel CRISPR/Cas14a system integrated with 2D porphyrin metal-organic framework for microcystin-LR determination through a homogeneous competitive reaction

Selective and sensitive detection of microcystin-LR (MC-LR) is of vital importance because of its high toxicity and broad distribution. Herein, a novel and versatile fluorescence sensor (Cas14-pMOFs fluorescence sensor) was developed by combining the CRISPR/Cas14a system with a 2D porphyrin metal-organic framework nanosheets (2D-pMOFs) for MC-LR determination. The designed CRISPR/Cas14a system was activated by the unbound complementary DNA (cDNA), which was positively correlated with MC-LR concentration. Furthermore, the activated Cas14a protein was utilized to indiscriminately cleave the FAM-labeled single-stranded DNA (ssDNA-FAM), which was pre-absorbed on Cu-TCPP(Fe) nanosheets. Because of the desorption of the cleaved ssDNA-FAM, the pre-quenched fluorescence signal was recovered. Owing to the excellent performance in quantifying cDNA using this Cas14-pMOFs fluorescence sensor with a limit of detection (LOD) of 0.12 nM, this Cas14-pMOFs fluorescence sensor was able to detect MC-LR in a range from 50 pg/mL to 1 μg/mL with the LOD of 19 pg/mL. This work not only provided a new insight for the exploration of fluorescence sensors based on 2D-pMOFs coupled with CRISPR/Cas14a, but also, demonstrated its universality in both nucleic acid and non-nucleic acid targets determination.

Improving Biosensors by the Use of Different Nanomaterials: Case Study with Microcystins as Target Analytes

The eutrophication of lakes and rivers without adequate rainfall leads to excessive growth of cyanobacterial harmful algal blooms (CyanoHABs) that produce toxicants, green tides, and unpleasant odors. The rapid growth of CyanoHABs owing to global warming, climate change, and the development of rainforests and dams without considering the environmental concern towards lakes and rivers is a serious issue. Humans and livestock consuming the toxicant-contaminated water that originated from CyanoHABs suffer severe health problems. Among the various toxicants produced by CyanoHABs, microcystins (MCs) are the most harmful. Excess accumulation of MC within living organisms can result in liver failure and hepatocirrhosis, eventually leading to death. Therefore, it is essential to precisely detect MCs in water samples. To date, the liquid chromatography-mass spectrometry (LC-MS) and enzyme-linked immunosorbent assay (ELISA) have been the standard methods for the detection of MC and provide precise results with high reliability. However, these methods require heavy instruments and complicated operation steps that could hamper the portability and field-readiness of the detection system. Therefore, in order for this goal to be achieved, the biosensor has been attracted to a powerful alternative for MC detection. Thus far, several types of MC biosensor have been proposed to detect MC in freshwater sample. The introduction of material is a useful option in order to improve the biosensor performance and construct new types of biosensors. Introducing nanomaterials to the biosensor interface provides new phenomena or enhances the sensitivity. In recent times, different types of nanomaterials, such as metallic, carbon-based, and transition metal dichalcogenide-based nanomaterials, have been developed and used to fabricate biosensors for MC detection. This study reviews the recent advancements in different nanomaterial-based MC biosensors.

Assembling of anisotropic plasmonic sheet-core-satellites for simultaneous ultrasensitive detection of MC-LR toxin

An anisotropic plasmonic sheet-core-satellite (PSCS) superstructure can be controlled via competitive binding between aptamer/MC-LR conjugation and aptamer-ssDNA hybridization. SERS nanotags can be incorporated into anisotropic plasmonic sheet-cores, e.g., pGO/nanorods, or pGO/hollow AgCl : Au nanoplates so as to fabricate an aptasensor for "ON-OFF" detection of MC-LR toxin. Preparing a PSCS superstructure and detection of toxin can be simultaneously completed so as to simplify the detection procedure of MC-LR toxin. Detection sensitivity of MC-LR toxin can be optimized by controlling aspect ratios or hollow interiors of plasmonic core nanoparticles. Herein, a limit of detection (0.635 pM) with a wide linear range from 1 pM to 10 nM can be obtained via optimized PSCS of pGO/nanorod/dotnanotags. When the aptasensor was tested in real samples, the PSCS shows excellent recoveries from 96.6% to 104.5% with relative standard deviation (RSD) lower than 2.89% in spiked reservoir samples. It can be predicted that a one-step facile nanofabrication/aptasensing approach would be extensively applied for rapid detection of some other environmental contaminants.

Cell density-dependent regulation of microcystin synthetase genes (mcy) expression and microcystin-LR production in Microcystis aeruginosa that mimics quorum sensing

As the secondary metabolites of cyanobacterial harmful algal blooms (Cyano-HABs), microcystins (MCs) were generated under various environmental and cellular conditions. The understanding of the causes of MCs generation is of great interest in the field of water treatment and environmental science. In this work, we studied how Microcystis aeruginosa (FACHB-905) cell densities affect the MCs synthetase genes (mcy) expression, microcystin-LR (MC-LR) and quorum sensing molecules (Acyl-homoserine lactones (AHLs)) production. An electrochemical sensor was developed here for sensitive and quantitative detection of MC-LR that cultured at different cell densities. The results showed that mcy expression and MC-LR concentration started to increase when the cell density reached ca. 22 × 10⁶ cells/mL, and was significantly increased with increasing cell densities. Moreover, the up-regulation of AHLs with increasing cell densities revealed that MC-LR is quorum sensing-mediated. Our results undoubtedly confirmed that MC-LR was produced in a cell density-dependent way that mimics quorum sensing, and the minimum cell density (ca. 22 × 10⁶ cells/mL) that was required to produce MC-LR was provided and offered a reference standard for the prevention and control of MCs pollution in the actual water environment.

Emerging Optical Materials in Sensing and Discovery of Bioactive Compounds

Optical biosensors are used in numerous applications and analytical fields. Advances in these sensor platforms offer high sensitivity, selectivity, miniaturization, and real-time analysis, among many other advantages. Research into bioactive natural products serves both to protect against potentially dangerous toxic compounds and to promote pharmacological innovation in drug discovery, as these compounds have unique chemical compositions that may be characterized by greater safety and efficacy. However, conventional methods for detecting these biomolecules have drawbacks, as they are time-consuming and expensive. As an alternative, optical biosensors offer a faster, simpler, and less expensive means of detecting various biomolecules of clinical interest. In this review, an overview of recent developments in optical biosensors for the detection and monitoring of aquatic biotoxins to prevent public health risks is first provided. In addition, the advantages and applicability of these biosensors in the field of drug discovery, including high-throughput screening, are discussed. The contribution of the investigated technological advances in the timely and sensitive detection of biotoxins while deciphering the pathways to discover bioactive compounds with great health-promoting prospects is envisaged to meet the increasing demands of healthcare systems.

Selective and easy detection of microcystin-LR in freshwater using a bioactivated sensor based on multiwalled carbon nanotubes on filter paper

Microcystin-LR (MC-LR) is a cyanobacterial toxin produced as a result of eutrophication in polluted water in warm weather conditions. The MC-LR could cause health problems in mammal organs such as the liver, heart, and muscle. Therefore, the World Health Organization (WHO) has stipulated a limit of <1.0 ng/mL in drinking water. Thus, detection and quantification are vital, but current techniques require complex and expensive offsite analysis. We have developed an inexpensive, sensitive, and field-deployable sensor based on bioactivated multiwalled carbon nanotubes (MWCNTs, diameter 20 nm) and micropore filter paper (0.45-μm pore size) for the detection of MC-LR. A specially designed DNA oligonucleotide (5-NH₂-C₆-AN₆) was used as the MC-LR targeting aptamer (MCTA). For bioactivation, MCTA was immobilized on the carboxylated MWCNTs via the formation of amide bonds. The bioactivated MWCNTs were deposited on the micropore filter paper by suction filtering. The detection of MC-LR in freshwater was possible within 1.5 h, achieved by measuring the changes in electrical resistance caused by the selective MC-LR and MCTA interactions. Despite suffering from some matrix effects, the detection limit of the sensor was 0.19 ng/mL for low-concentration MC-LR (≤0.5 ng/mL). This method is much cheaper (biosensor price: < $2.5) than liquid chromatography coupled with tandem mass spectroscopy analysis (ca. $50/sample) which is a standard method for MC-LR detection in a modern laboratory. Thus, this cheap and straightforward MC-LR sensor has applications for detection in remote locations.

Real-Time Intraoperative Surface-Enhanced Raman Spectroscopy-Guided Thermosurgical Eradication of Residual Microtumors in Orthotopic Breast Cancer

Residual microtumors following surgical resection are the major cause for lethal cancer recurrence. However, it remains challenging to completely eliminate these residual microtumors. Here, we report an integrated strategy for image-guided surgical resection of tumors and intraoperative surface-enhanced Raman spectroscopy (SERS) guided thermosurgical elimination of residual microtumors using a "three-in-one" theranostic nanoprobe, termed the Au nanostar-based photoacoustic (PA), SERS, and thermosurgical (starPART) probe. This starPART probe, comprising an Au nanostar core, a Raman molecule layer, and a silica outer layer, draws upon the significant advantages of PA imaging, SERS detection, and photothermal tumor ablation. These prominent features enable preoperative PA imaging for surgical resection of tumors and intraoperative SERS-guided thermosurgery for complete elimination of residual microtumors. In vivo experiments confirm complete eradication of microtumors without local recurrence and with a 100% tumor-free survivability. This work therefore offers a robust platform for real-time intraoperative eradication of residual microtumors with significant improvement of surgical outcomes.

Interfacial interactions of SERS-active noble metal nanostructures with functional ligands for diagnostic analysis of protein cancer markers

Noble metal nanostructures with designed hot spots have been widely investigated as surface-enhanced Raman spectroscopy (SERS)-active substrates, particularly for selective and sensitive detection of protein cancer markers. For specific target recognition and efficient signal amplification, SERS probe design requires a choice of SERS-active nanostructures as well as their controlled functionalization with Raman dyes and target recognition entities such as antibodies. However, the chemical conjugation of antibodies and Raman dyes to SERS substrates has rarely been discussed to date, despite their substantial roles in detection schemes. The interfacial interactions of metal nanostructures with functional ligands during conjugation are known to be strongly influenced by the various chemical and physical properties of the ligands, such as size, molecular weight, surface charge, 3-dimensional structures, and hydrophilicity/hydrophobicity. In this review, we discuss recent developments in the design of SERS probes over the last 4 years, focusing on their conjugation chemistry for functionalization. A strong preference for covalent bonding is observed with Raman dyes having simpler molecular structures, whereas more complicated ones are non-covalently adsorbed. Antibodies are both covalently and non-covalently bonded to nanostructures, depending on their activity in the SERS probes. Considering that ligand conjugation is highly important for chemical stability, biocompatibility, and functionality of SERS probes, this review is expected to expand the understanding of their interfacial design, leading to SERS as one of the most promising spectroscopic analytical tools for the early detection of protein cancer markers.

Multiplexed SERS Detection of Microcystins with Aptamer-Driven Core-Satellite Assemblies

We describe surface-enhanced Raman spectroscopy (SERS) aptasensors that can indirectly detect MC-LR and MC-RR, individually or simultaneously, in natural water and in algal culture. The sensor is constructed from nanoparticles composed of successive layers of Au core-SERS label-silver shell-gold shell (Au@label@Ag@Au NPs), functionalized on the outer Au surface by MC-LR and/or MC-RR aptamers. These NPs are immobilized on asymmetric Au nanoflowers (AuNFs) dispersed on planar silicon substrates through DNA hybridization of the aptamers and capture DNA sequences with which the AuNFs are functionalized, thereby forming core-satellite nanostructures on the substrates. This construction led to greater electromagnetic (EM) field enhancement of the Raman label-modified region, as supported by finite-difference time-domain (FDTD) simulations of the core-satellite assembly. In the presence of MC-LR and/or MC-RR, the aptamer-functionalized NPs dissociate from the AuNFs because of the stronger affinity of the aptamers with the MCs, which decreases the SERS signal, thus allowing indirect detection of the MCs. The improved SERS sensitivity significantly decreased the limit of detection (LOD) for separate MC-LR detection (0.8 pM) and for multiplex detection (1.5 pM for MC-LR and 1.3 pM for MC-RR), compared with other recently reported SERS-based methods for MC-LR detection. The aptasensors show excellent selectivity to MC-LR/MC-RR and excellent recoveries (96-105%). The use of these SERS aptasensors to monitor MC-LR production over 1 week in a culture medium of M. aeruginosa cells demonstrates the applicability of the sensors in a realistic environment.

Quantitative PCR based detection system for cyanobacterial geosmin/2-methylisoborneol (2-MIB) events in drinking water sources: Current status and challenges

Taste and odor (T&O) are an important issue in drinking water, aquaculture, recreation and a few other associated industries, and cyanobacteria-relevant geosmin and 2-methylisoborneol (2-MIB) are the two most commonly detected T&O compounds worldwide. A rise in the cyanobacterial blooms and associated geosmin/2-MIB episodes due to anthropogenic activities as well as climate change has led to global concerns for drinking water quality. The increasing awareness for the safe drinking, aquaculture or recreational water systems has boost the demand for rapid, robust, on-site early detection and monitoring system for cyanobacterial geosmin/2-MIB events. In past years, research has indicated quantitative PCR (qPCR) as one of the promising tools for detection of geosmin/2-MIB episodes. It offers advantages of detecting the source organism even at very low concentrations, distinction of odor-producing cyanobacterial strains from non-producers and evaluation of odor producing potential of the cyanobacteria at much faster rates compared to conventional techniques.The present review aims at examining the current status of developed qPCR primers and probes in identifying and detecting the cyanobacterial blooms along with geosmin/2-MIB events. Among the more than 100 articles about cyanobacteria associated geosmin/2-MIB in drinking water systems published after 1990, limited reports (approx. 10 each for geosmin and 2-MIB) focused on qPCR detection and its application in the field. Based on the review of literature, a comprehensive open access global cyanobacterial geosmin/2-MIB events database (CyanoGM Explorer) is curated. It acts as a single platform to access updated information related to origin and geographical distribution of geosmin/2-MIB events, cyanobacterial producers, frequency, and techniques associated with the monitoring of the events. Although a total of 132 cyanobacterial strains from 21 genera and 72 cyanobacterial strains from 13 genera have been reported for geosmin and 2-MIB production, respectively, only 58 geosmin and 28 2-MIB synthesis regions have been assembled in the NCBI database. Based on the identity, geosmin sequences were found to be more diverse in the geosmin synthase conserved/primer design region, compared to 2-MIB synthesis region, hindering the design of universal primers/probes. Emerging technologies such as the bioelectronic nose, Surface Enhanced Raman Scattering (SERS), and nanopore sequencing are discussed for future applications in early on-site detection of geosmin/2-MIB and producers. In the end, the paper also highlights various challenges in applying qPCR as a universal system of monitoring and development of response system for geosmin/2-MIB episodes.

A Mini-Review on Detection Methods of Microcystins

Cyanobacterial harmful algal blooms (CyanoHABs) produce microcystins (MCs) which are associated with animal and human hepatotoxicity. Over 270 variants of MC exist. MCs have been continually studied due of their toxic consequences. Monitoring water quality to assess the presence of MCs is of utmost importance although it is often difficult because CyanoHABs may generate multiple MC variants, and their low concentration in water. To effectively manage and control these toxins and prevent their health risks, sensitive, fast, and reliable methods capable of detecting MCs are required. This paper aims to review the three main analytical methods used to detect MCs ranging from biological (mouse bioassay), biochemical (protein phosphatase inhibition assay and enzyme linked immunosorbent assay), and chemical (high performance liquid chromatography, liquid chromatography-mass spectrometry, high performance capillary electrophoresis, and gas chromatography), as well as the newly emerging biosensor methods. In addition, the current state of these methods regarding their novel development and usage, as well as merits and limitations are presented. Finally, this paper also provides recommendations and future research directions towards method application and improvement.

A Fluorescence and Surface-Enhanced Raman Spectroscopic Dual-Modal Aptasensor for Sensitive Detection of Cyanotoxins

The ability to detect trace analytes without necessitating solid surface attachment or complicated processing steps would facilitate the translation of sensors for monitoring environmental toxins in the field. To address a critical unmet need in fresh water ecology, we have developed a dual-modal aptamer-based biosensor (aptasensor), featuring fluorescence and surface-enhanced Raman spectroscopy (SERS), for sensitive and selective detection of hepatotoxin microcystin-LR (MC-LR). The rational sensor design is based on the high affinity of the cyanine (Cy3) dye-modified complementary DNA (Cy3-cDNA) strand toward the plasmonic gold nanostars (GNSs) in comparison to the Cy3-cDNA/aptamer duplex. The preferential binding of MC-LR toward the MC-LR-specific aptamer triggers the dissociation of Cy3-cDNA/aptamer duplexes, which switches the Cy3's fluorescence "off" and SERS "on" due to the proximity of Cy3 dye to the GNS surface. Both fluorescence and SERS intensities are observed to vary linearly with the MC-LR concentration over the range of investigation. We have achieved high sensitivity and excellent specificity with the aptasensor toward MC-LR, which can be attributed to the fluorescence quenching effect, significant SERS enhancement by the GNSs, and the high affinity of the aptamer toward the MC-LR analytes. We further demonstrate the applicability of the present aptasensor for detection of MC-LR in a diverse set of real water samples with high accuracy and excellent reproducibility. With further refinement, we believe that the aptamer-driven complementary assembly of the SERS and fluorescence sensing constructs can be applied for rapid, multiplexed, and robust measurements of environmental toxins in the field.

Molecular-Imprinting-Based Surface-Enhanced Raman Scattering Sensors

Molecularly imprinted polymers (MIPs) receive extensive interest, owing to their structure predictability, recognition specificity, and application universality as well as robustness, simplicity, and inexpensiveness. Surface-enhanced Raman scattering (SERS) is regarded as an ideal optical detection candidate for its unique features of fingerprint recognition, nondestructive property, high sensitivity, and rapidity. Accordingly, MIP based SERS (MIP-SERS) sensors have attracted significant research interest for versatile applications especially in the field of chemo- and bioanalysis, showing excellent identification and detection performances. Herein, we comprehensively review the recent advances in MIP-SERS sensors construction and applications, including sensing principles and signal enhancement mechanisms, focusing on novel construction strategies and representative applications. First, the basic structure of the MIP-SERS sensors is briefly outlined. Second, novel imprinting strategies are highlighted, mainly including multifunctional monomer imprinting, dummy template imprinting, living/controlled radical polymerization, and stimuli-responsive imprinting. Third, typical application of MIP-SERS sensors in chemo/bioanalysis is summarized from both small and macromolecular aspects. Lastly, the challenges and perspectives of the MIP-SERS sensors are proposed, orienting sensitivity improvement and application expanding.

Preparation of Aptamer Responsive DNA Functionalized Hydrogels for the Sensitive Detection of α-Fetoprotein Using SERS Method

Hepatocellular carcinoma (HCC), which is one of the three major cancers, has attracted growing attention due to its high mortality, health care cost, and circumscribed therapeutic methods. Hence, the development of a fast, accurate, and flexible method to detect α-fetoprotein (AFP), the specific marker of HCC, is significant for diagnosis and treatment of cancer. Here, we constructed a novel SERS biosensing platform combining the target-responsive DNA hydrogel for the sensitive detection of AFP. The linker strand in DNA hydrogel is an aptamer that can specifically recognize AFP and accurately control the release of immunoglobulin G (IgG) encapsulated in hydrogel. In the presence of AFP, the hydrogels were disentangled and the IgG was released. Thereafter, the released IgG was captured by SERS probes and biofunctional magnetic beads through formation of sandwich-like structures, resulting in the signal of Raman tags decreasing in the supernatant after magnetic separation. Due to the ultrahigh sensitivity of the SERS biosensor, the proposed method has a wide detection linear range (50 pg/mL to 0.5 μg/mL) and a detection limit down to 50 pg/mL. Moreover, the sequence of the linker strand in the DNA hydrogel can be specifically encoded into a new aptamer that responds to other cancer markers. This convenient and inexpensive detection method provides a new strategy for the detection of tumor markers.