DNA-templated fluorescent silver nanoclusters on-off switch for specific and sensitive determination of organic mercury in seafood

Recent progress in nanomaterial-based aptamers as biosensors for point of care detection of Hg2+ ions and its environmental applications

Among the foremost persistent heavy metal ions in the ecosystem, mercury (Hg2+) remains intimidating to the environment by producing a catastrophic effect on the environment as well as on mankind due to the exacerbation of anthropogenic activities. Therefore, it has become necessary to develop superlative techniques for its detection even at low concentrations. The conventional approaches for Hg2+ ions are quite laborious, and expensive, and require expertise in operating sophisticated instruments. To overcome these limitations, aptamer-based biosensors emerged as a promising tool for its detection. DNA-based aptamers have evolved as a significant technique by detecting them even in ppb levels. This review outlines the progress in aptamer-based biosensors from the year 2019-2023 by inducing changes in the electrochemical signal or by fluorescent/colorimetric approaches. The electrochemical sensors used nanomaterial electrodes for increasing the sensitivity whereas fluorescent and colorimetric sensors exhibit quenching or strong fluorescence in the presence of Hg2+ ions depending upon the prevailing mechanism or visible color changes. This perturbation in the signals could be attributed to the formation of the T-Hg2+ -T complex with the aptamers in the presence of ions revealing its real-time and biological applications in living or cancerous cells. Furthermore, next-generation biosensors are suggested to bring a paradigm shift to the integration of high-end smartphones, machine learning, artificial intelligence, etc.

Fluorescent DNA-Silver nanoclusters in food safety detection: From synthesis to application

In recent years, the conventional preparation of silver nanoclusters (AgNCs) has attracted much attention due to their ultra-small size, tunable fluorescence, easy-to-engineer, as well as biocompatible material. Moreover, its great affinity towards cytosine bases on single-stranded DNA has led to the construction of biosensors, especially aptamers, for a broad variety of applications in food safety and environmental protection. In past years, numerous researchers paid attention to the construction of AgNCs aptasensor. Therefore, this review will be an effort to summarize the synthetic strategy along with the influences of factors on synthesis, categorize the sensing mechanism of aptamer-functionalized AgNCs biosensors, as well as their specific applications in food safety detection including heavy metal, toxin, and foodborne pathogenic bacteria. Furthermore, a brief conclusion and outlook regarding the prospects and challenges of their applications in food safety were drawn in line with the developments in DNA-AgNCs.

Nanotechnology-based analytical techniques for the detection of contaminants in aquatic products

Food safety of aquatic products has attracted considerable attention worldwide. Although a series of conventional bioassays and instrumental methods have been developed for the detection of pathogenic bacteria, heavy metal residues, marine toxins, and biogenic amines during the production and storage of fish, shrimp, crabs et al., the nanotechnology-based analyses still have their advantages and are promising since they are cost-efficient, highly sensitive and selective, easy to conduct, facial design, often require no sophisticated instruments but with excellent detection performance. This review aims to summarize the advances of various biosensing strategies for bacteria, metal ions, and small molecule contaminants in aquatic products during the last five years, The review highlights the development in nanotechnologies applied for biorecognition process, signal transduction and amplification methods in each novel approach, the nuclease-mediated DNA amplification, nanomaterials (noble metal nanoparticle, metal-organic frameworks, carbon dots), lateral flow-based biosensor, surface-enhanced Raman scattering, microfluidic chip, and molecular imprinting technologies were especially emphasized. Moreover, this study provides a view of current accomplishments, challenges, and future development directions of nanotechnology in aquatic product safety evaluation.

Silver nanocluster-based aptasensor for the label-free and enzyme-free detection of ochratoxin A

Mycotoxin contamination in cereal is a global concern, threatening food safety and human health, necessitating the development of rapid on-site methods. Here, a label- and enzyme-free biosensor was developed based on aptamer-regulated DNA-silver nanoclusters (AgNCs) for rapid detection of ochratoxin A (OTA). A novel DNA-templated AgNCs emitting strong red fluorescence was designed and synthesized in this study. The partial sequence of the DNA template was selected from the complementary OTA aptamer (Apt-OTA) sequence, which can quench fluorescence from the AgNCs via hybridization in the absence of OTA. In the presence of OTA, the high OTA-Aptamer affinity prevented the Apt-OTA from quenching the AgNCs, resulting in "turn on" of the fluorescence. This biosensor eliminated the use of costly reagents, complex pretreatments, and sophisticated equipment, which could realize the point-of-care testing (POCT) of OTA with a limit of detection (LOD) of 1.3 nM and a detection time of 45 min.

A broad-applicability method for mercury speciation in various seafoods using microwave-assisted extraction and ion chromatography-inductively coupled plasma mass spectrometry

Almost all marine organisms contain both inorganic and organic mercury, and thus it is extremely important to determine mercury species in seafood to objectively and scientifically assess the health risk posed by mercury. We herein developed a broad-applicability microwave-assisted extraction method and a robust ion chromatography-inductively coupled plasma mass spectrometry (IC-ICP-MS) method for the speciation analysis of mercury in various seafood samples including seaweeds, fishes and shellfishes. The extraction method has broad adaptability, it can be used to simultaneously extract mercury species from various seafood samples including seaweeds, fishes and shellfishes without altering the chemical species of mercury, with an extraction efficiency >90%. Especially, the seafood extract obtained with the extraction method can be directly used for the following IC-ICP-MS determination of mercury species without additional pretreatment. The IC-ICP-MS method used low-cost cation guard columns as the separation column, and has an instrument detection limit of 0.02-0.05 ng mL^-1 for Hg²⁺, CH₃Hg⁺ and C₂H₅Hg⁺. The developed extraction and IC-ICP-MS methods have been successfully used to determine Hg²⁺, CH₃Hg⁺ and C₂H₅Hg⁺ in various seaweeds, fishes and shellfishes without the matrix effect, with a method detection limit of 2.4-6.0 ng g^-1 dried weight, a recovery of 92-105%, and a relative standard deviation (RSD, n = 5) of less than or equal to 6%. The success of this study offers a reliable and universal approach for the speciation analysis of mercury in seafood, which may provide the database for objectively assessing the health risks of mercury in seafood and ensuring the safety of consumption of seafood.

Aptamer-based NanoBioSensors for seafood safety

Chemical and biological contaminants are of primary concern in ensuring seafood safety. Rapid detection of such contaminants is needed to keep us safe from being affected. For over three decades, immunoassay (IA) technology has been used for the detection of contaminants in seafood products. However, limitations inherent to antibody generation against small molecular targets that cannot elicit an immune response, along with the instability of antibodies under ambient conditions greatly limit their wider application for developing robust detection and monitoring tools, particularly for non-biomedical applications. As an alternative, aptamer-based biosensors (aptasensors) have emerged as a powerful yet robust analytical tool for the detection of a wide range of analytes. Due to the high specificity of aptamers in recognising targets ranging from small molecules to large proteins and even whole cells, these have been suggested to be viable molecular recognition elements (MREs) in the development of new diagnostic and biosensing tools for detecting a wide range of contaminants including heavy metals, antibiotics, pesticides, pathogens and biotoxins. In this review, we discuss the recent progress made in the field of aptasensors for detection of contaminants in seafood products with a view of effectively managing their potential human health hazards. A critical outlook is also provided to facilitate translation of aptasensors from academic laboratories to the mainstream seafood industry and consumer applications.

Sensitive detection and intracellular imaging of free copper ions based on DNA-templated silver nanoclusters aggregation-inducing fluorescence enhancement effect

Free copper ions (Cu⁺ and Cu²⁺) have critical toxicity to cells, although copper is an essential element for human body. Hence, sensitive monitoring is crucial to avoid over intake of Cu⁺/Cu²⁺. We herein designed a ssDNA sequence (A₃₁) for synthetizing A₃₁-templated silver nanoclusters (AgNCs), and demonstrated that Cu⁺/Cu²⁺ can induce the aggregation of A₃₁-templated AgNCs and thus greatly enhanced the fluorescence emission of A₃₁-templated AgNCs. Based on Cu⁺/Cu²⁺-induced fluorescence enhancement effect of A₃₁-templated AgNCs, a label-free and signal-on fluorescent sensing platform was developed for the specific and sensitive detection of Cu⁺/Cu²⁺ in biological samples and intracellular imaging of Cu⁺/Cu²⁺ in cells. The signal-on fluorescent sensing platform could be used to rapidly detect Cu⁺ and Cu²⁺ with a detection limit of 0.1 µM within 30 min., and to perform the intracellular imaging of Cu⁺ and Cu²⁺ in cells with good cell permeability and biocompatibility. By using the signal-on fluorescent sensing platform, we have successfully detected Cu⁺ and Cu²⁺ in cells fluids and human serum with a recovery of 90-104% and a RSD (n = 5) < 5%, and performed the imaging of Cu⁺/Cu²⁺ in Hela cells. The developed fluorescent sensing platform has obvious analytical and imaging advantages such as signal-on, simple operation, short analysis time, both Cu⁺ and Cu²⁺ detection, similar or higher sensitivity, good cell permeability and biocompatibility, which promising a reliable approach for the rapid and on-site detection or imaging of free copper ions in biological samples in clinical diagnosis.

Synthesis of Xylan-Click-Quaternized Chitosan via Click Chemistry and Its Application in the Preparation of Nanometal Materials

For the high-valued utilization of hemicelluloses and for realizing the controllable synthesis of NPs, this paper's aim is to combine xylan, chitosan and nanometal materials at the same time. In this research study, firstly, propargyl xylan was synthesized via nucleophilic substitution reaction between xylan and propargyl bromide in NaOH solution. On the other hand, a tosyl group was introduced onto the 6th position of synthesized quaternized chitosan (QCS), and the azide group replaced the tosyl group to obtain 6-amido-QCS (QCS-N3). The synthesis conditions of the above reactions were optimized. Subsequently, the novel xylan-click-QCS polymer was obtained via click reaction between terminal alkyne groups on the xylan chains and azide groups on QCS. Then, AgNPs and AuNPs were synthesized by adopting the xylan-click-QCS polymer as the reducing and stabilizing agent, and the reaction conditions were optimized to obtain well-dispersed and highly stable nanoparticles. There were two kinds of Ag nanomaterials, with diameters of 10~20 nm and 2~5 nm, respectively, indicating the formation of Ag nanoclusters, except for Ag nanoparticles, in this reaction. The diameter of the synthesized AuNPs was 20~30 nm, which possessed a more uniform size distribution. The Ag nanoclusters with a smaller size (2~5 nm) could inhibit MCF-7 cell proliferation effectively, indicating their application potential in cancer therapy. The study gives a new approach to the high-value utilization of biopolymers.

Amplifiable ratiometric fluorescence biosensing of nanosilver multiclusters populated in three-way-junction DNA branches

To explore the fluorescence bio-responsiveness of emissive silver nanoclusters (AgNCs) populated in DNA-branched scaffolds is intriguing yet challenging. In response to a desired targeting model (T*) as a vehicle, herein a customized three-way-junction DNA construct (TWJDC) is assembled via competitive hybridizing cascade among three stem-loop hairpins with specific base sequences, where the repeated recycling of T* enables the exponentially amplifiable output of rigid TWJDC. As designed, these stable hybridization products are highly T*-stimulated responsive and constructing-directional. In the three branched-arms, the unpaired sticky ends provide isotropic binding sites for a signaling hairpin encoded with two C-rich templates of green- and red-AgNCs clustering. The identical ligation of signal probe with three arms of TWJDC liberates its locked stem, enabling the separate growth of red-clusters in three branches. As demonstrated, three clusters of red-AgNCs possess advantageous self-enhancing fluorescent performance relative to single or two cluster(s), good biocompatibility and low cytotoxicity. Utilizing the bicolor AgNCs as dual-emitters with reversely changed emission intensity, we developed an innovative ratiometric strategy displaying sensitively linear dose-dependence on variable T* down to 1.9 pM, which can afford a promising platform for biosensing, bioanalysis, cell imaging, or even clinical theranostics.

Recent Advances on Functional Nucleic-Acid Biosensors

In the past few decades, biosensors have been gradually developed for the rapid detection and monitoring of human diseases. Recently, functional nucleic-acid (FNA) biosensors have attracted the attention of scholars due to a series of advantages such as high stability and strong specificity, as well as the significant progress they have made in terms of biomedical applications. However, there are few reports that systematically and comprehensively summarize its working principles, classification and application. In this review, we primarily introduce functional modes of biosensors that combine functional nucleic acids with different signal output modes. In addition, the mechanisms of action of several media of the FNA biosensor are introduced. Finally, the practical application and existing problems of FNA sensors are discussed, and the future development directions and application prospects of functional nucleic acid sensors are prospected.

Dual-Emission Fluorescence Probe Based on CdTe Quantum Dots and Rhodamine B for Visual Detection of Mercury and Its Logic Gate Behavior

It is urgent that a convenient and sensitive technique of detecting Hg2+ be developed because of its toxicity. Conventional fluorescence analysis works with a single fluorescence probe, and it often suffers from signal fluctuations which are influenced by external factors. In this research, a novel dual-emission probe assembled through utilizing CdTe quantum dots (QDs) and rhodamine B was designed to detect Hg2+ visually. Only the emission of CdTe QDs was quenched after adding Hg2+ in the dual-emission probe, which caused an intensity ratio change of the two different emission wavelengths and hence facilitated the visual detection of Hg2+. Compared to single emission QDs-based probe, a better linear relationship was shown between the variation of fluorescence intensity and the concentration of Hg2+, and the limit of detection (LOD) was found to be11.4 nM in the range of 0-2.6 μM. Interestingly, the intensity of the probe containing Hg2+ could be recovered in presence of glutathione (GSH) due to the stronger binding affinity of Hg2+ towards GSH than that towards CdTe QDs. Based on this phenomenon, an IMPLICATION logic gate using Hg2+/GSH as inputs and the fluorescence signal of QDs as an output was constructed.