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The monitoring of small-molecule thiols (especially glutathione) has attracted widespread attention due to their involvement in numerous physiological processes in living organisms and cells. In this work, a dual-mode nanosensor was designed to detect small-molecule thiols, which is based on the "on-off" switch of fluorescence resonance energy transfer (FRET) and surface-enhanced Raman scattering (SERS). Briefly, DNA was modified by Cy5 (signal probe) and disulfide bonds (recognition element). Gold nanoflowers (AuNFs) were used as the fluorescence-quenching and SERS-enhancing substrate. However, small-molecule thiols can cleave disulfide bonds and release short Cy5-labeled chains, causing the recovery of the fluorescence signal and a decrease of the SERS signal. The nanosensor showed a sensitive response to small-molecule thiols represented by GSH, with a linear range of 0.01-3 mM and a detection limit of 913 nM. In addition, it competed with other related biological interferences and presented good stability and better selectivity towards small-molecule thiols. Most importantly, the developed nanosensor had been successfully applied to in situ imaging and quantitative monitoring of the concentration of small-molecule thiols which changed during T-2 toxin-induced apoptosis in HeLa cells. Meanwhile, nanosensors are also versatile with their potential applications and can be easily extended to the detection and imaging of other human cell lines. The proposed method combines the dual advantages of fluorescence and SERS, which has broad prospects for in situ studies of physiological processes involving small-molecule thiols in biological systems.

Fluorescent biosensors and imaging devices have gained fervent consideration due to their prime functionality in biological. Among fluorescent nanomaterial (FNMs), the ultra-small gold nanoclusters (AuNCs) have gained promising attention with respect to extra-ordinary properties of bright fluorescence, economical synthesis, higher photostability, and biocompatibility, and deep tissue penetration. Therefore, the prior decades comprehended the revolutions in the field of real-time monitoring devices, nanotechnology-based biosensing, and bioimaging sensors. The present review primarily focuses on metal NCs (MNCs) and their advantages, a brief introduction of AuNCs along with history prospective and development, fundamental aspects regarding AuNCs quality and fluorescence, limitations and advantages of AuNCs, sensing mechanism, expected synthesis principle, and summarized the recent progress of AuNCs probe over the last 3 years (2019–2021) with their respective detection and bioimaging mechanism and synthesis principle. Moreover, the present work also serves as a novel stratagem for the preparation and potential applications of multifunctional AuNCs nano-systems responsiveness for various analytes detection and cell bioimaging with respective examples. At last, we described the challenges associated with the application of AuNCs based on recent signs of progress. [Display omitted] • This review briefly concise the AuNCs sensor with respective to their sensing principal. • AuNCs as emerging analytical tool are new paradigm in sensing and bioimaging. • Summarize the progress and efforts endowed by the researchers in last three years (2019–2021). • AuNCs open new avenue to serve practical platform as for scientists to explore and paint for wide range of application. [ABSTRACT FROM AUTHOR]

The performance of chloramphenicol aptamer, including binding thermodynamics, structure switching, and binding domain, was investigated by isothermal titration calorimetry, circular dichroism, and molecular docking. Then, a new fluorescence aptasensor was developed with signal amplification mediated by exonuclease I-catalyzed reaction and hybridization chain reaction (HCR) for chloramphenicol detection. In this system, the aptamer-binding domain is blocked by the initiator of HCR, the aptamer undergoes structure switching in the presence of chloramphenicol, and DNA dissociation occurs. The released aptamer is subsequently recognized and cleaved by Exo I to set free chloramphenicol. With the Exo I-assisted chloramphenicol recycling, an increasing number of initiators were exposed from the digestion of the initiator-aptamer complex. Then, the chain-like assembly of FAM labeled H1 and H2 through HCR was triggered by the initiator, generating a long DNA polymer. Under optimum conditions, the aptasensor exhibited a log-linear range from 0.001 to 100 nM of chloramphenicol and a detection limit of 0.3 pM. Additionally, the designed biosensing platform was applied to determine chloramphenicol in milk and lake water with high accuracy. The current approach provides a new avenue to develop sensitive aptasensors with the assistance of binding mechanism between aptamer and target compounds. [ABSTRACT FROM AUTHOR]

• A novel time-resolved fluorescence aptasensor for detection of AFM 1 in milk, was developed by coupling g-C 3 N 4 nanosheet and rolling circle amplification based-DNA amplification. • The time-resolved fluorescence nanoparticles functionalized with cDNA was used as signal probe and graphitic carbon nitride nanosheet was used as a quencher for fluorescence. • The LOD of AFM 1 was 0.0194 pg/mL, much lower than earlier described methods and has good recoveries between 92 and 99.8 %. Aflatoxin M 1 is a prevailing natural carcinogenic mycotoxin, found in milk. Herein, we reported a sensitive fluorescent aptasensor for AFM 1 , based on rolling circle amplification (RCA) technology to improve the sensitivity of the assay. The detection of AFM 1 was achieved using KYF 4 : Eu3+ time-resolved fluorescent nanoparticles (KYF 4 : Eu3+ TRFNPs) as signal probe and graphitic carbon nitride (g-C 3 N 4) nanosheet as a quencher. The AFM 1 aptamer act as a recognition element and as a primer for DNA synthesis. TRFNPs attached to complementary DNA (TRFNPs-cDNA) acts as a signal probe. In the absence of target, a rolling circle template (RCT) ligated to aptamer and RCA reaction initiated. The TRFNPs-cDNA was strengthened to amplified RCA product (RCAP) to make RCAP/TRFNPs-cDNA duplex. This duplex cannot be adsorbed onto g-C 3 N 4 , and ultimately no fluorescence quenched. In the presence of AFM 1 , aptamer recognized the target and aptamer-target complex was formed. Hence, no aptamer hybridized with RCT, therefore, no RCA and duplex formation. Consequently, free TRFNPs-cDNA adsorbed onto g-C 3 N 4 and resulted in quenched fluorescence. The assay provides a lower detection limit (0.0194 pg/mL) than the previously reported assays and has good recoveries between 92–99.8 %. Hence, the present method would be a suitable method for the quantification of small molecules. [ABSTRACT FROM AUTHOR]

The emergent arena of bio-nanotechnology aims at modernizing the detection and bioimaging discipline via the introduction of aggregation-induced emission (AIE)-based tools. AIE is a unique photophysical phenomenon because of the photoemission of the propeller-like molecules that could be significantly enhanced after aggregation. AIE differs immensely from the generally recognized aggregation-caused emission quenching (ACQ) observed in numerous conventional luminophores that own well-conjugated and planar structures. The luminogens with AIE aspects (AIEgens) are more promising to be applied in various research front. Driven by ever-increasing interests for good applicability, fast response, excellent fluorescence and sensitivity, the fluorescent AIE-probes with numerous working methodology and innovative functionalities are prospering at an astonishing speed. In this review, we have presented a brief introduction of AIE with development history, comparison with conventional fluorophores, expected working methodology, discuss the structure-property relationship of the AIEgens and summarized the recent progress of AIE molecular and nanomaterial probe over the past 3 years with their respective mechanism of detection and bioimaging. In addition, this work provides a novel platform for the preparation and potential applications of multifunctional AIE-active nano-systems responsiveness for various detection of analytes and cell bioimaging with respective examples and also encompasses the recent progress, challenges and potential breakthroughs. Image 1 • This review summarizes the various AIE based sensors, probes and molecules with respective to their sensing principal. • Analytical performances of AIE are new paradigm in field of detection and bioimaging. • Concise the efforts and progress made by the researchers in last three years (2017–2019). • AIE serve a practical platform to open a new avenue for scientists to explore and paint for wide range of application. [ABSTRACT FROM AUTHOR]

A DNA tweezers fluorescence aptasensor based on split aptamer assisted-magnetic nanoparticles (MNPs) and magnetic separation was established for the detection of Enrofloxacin (ENR). The DNA tweezers' fluorescence aptasensor consists of two arms with fluorophores (FAM) and quenchers (BHQ 1) connected at both ends. Meanwhile, the high fluorescence signal can be generated with FAM and BHQ 1 being spatially separated. Split aptamer 1 (SPA 1) of ENR was annealed and incubated with avidin-coated MNPs. ENR and split aptamer 2 (SPA 2) were added to form a sandwich structure with two split aptamer strands (MNPS-SPA 1 /ENR/SPA 2). After magnetic separation, the supernatant did not interact with DNA tweezers, which made DNA tweezers a "turn-on" aptasensor to enhance the fluorescence signal. In the absence of ENR, SPA 2 was free in the supernatant to dramatically quench the fluorescence which made DNA tweezers a "turn-off" aptasensor. The linear range of ENR detection was 0.01–100 ng/mL with a limit of detection of 0.008 ng/mL. The prepared DNA tweezers fluorescence aptasensors were applied to detect ENR in spiked milk samples, which showed good recoveries between 90.27% and 106.62%. In addition, the present method showed great potential for the target, employing a split aptamer detection platform in practical application. • A novel fluorescence aptasensor assay for Enrofloxacin (ENR) detection was established. • The sandwich structure is formed by specific target recognition of split aptamer to achieve sensitive detection. • The "turn-on/turn-off" structure of DNA tweezers offers significant fluorescence signals. [ABSTRACT FROM AUTHOR]

Herein, a surface-enhanced Raman scattering (SERS) and fluorescence dual-signal aptasensor has been developed for aflatoxin B 1 (AFB 1) detection. Fluorescence signal source CdTe quantum dots were decorated on the outer of silica nanoparticles and further functionalized with AFB 1 aptamer. Raman reporter molecules DTNB were decorated on the surface of gold nanoparticles (AuNPs) together with the aptamer complementary chains. The two parts of nanomaterials were combined by the base complementary pairing principle, to form satellite nanostructures with high SERS and low fluorescence signals following LSPR and FRET effect. The addition of AFB 1 led to the shedding of AuNPs due to the higher sensitivity of the aptamer towards the target and resulting in recovered fluorescence and decreased SERS signals. The developed method showed a linear relationship in the range of 10−4-103 ng/mL, with detection limit of 0.094 pg/mL. This technology had strong application prospects due to simplicity of operation and high accuracy. • The dual-signal aptasensor of AFB 1 was realized by combining LSPR and FRET effect. • The signals were amplified by aggregating the nanomaterials around the silica. • T he aptasensor s howed good accuracy in real samples and verified by HPLC. • The LOD of AFB 1 was as low as 0.094 pg/mL. [ABSTRACT FROM AUTHOR]

Glutathione (GSH) is an important intracellular antioxidant. It is closely related to the homeostasis of the intracellular redox state which can be changed through deoxynivalenol (DON) exposure. More importantly, GSH deficiency can directly lead to apoptosis and cellular metabolic disorders mediated by intracellular redox homeostasis. Therefore, real-time monitoring of dynamic changes of GSH levels in living cells is urgently needed for the evaluation of intracellular oxidative stress. Herein, for the first time, an aptamer-based recognition composite nanoprobe (Apt-AuNCs-MoS 2) was successfully constructed. The nanoprobe was employed for real-time and in situ imaging of DON-induced intracellular GSH changes and evaluation of oxidative stress status. The results indicated that the aptasensor revealed excellent specificity and high sensitivity to GSH in the solution system, and the limit of detection was as low as 35 nM. The results about DON-mediated oxidative stress indicated that the oxidative stress in living cells tends to be saturated with more than 20 ppm DON concentration. Thus, the strategy provides a novel aptasensor for the determination of GSH, offers a promising platform for fluorescence imaging of intracellular GSH in living cells, and further enriches in situ analysis of the in vitro toxicology. [Display omitted] • An aptasensor for GSH sensing is successfully constructed for the first time. • The aptasensor reveals excellent specificity and high sensitivity to GSH. • The aptasensor is employed for imaging GSH and evaluating DON-induced oxidative stress in living cells. • The aptasensor enriches in situ analysis methods for the in vitro toxicology. [ABSTRACT FROM AUTHOR]

The authors describe a fluorescence polarization (FP) aptasensor based on the polymerase chain reaction (PCR) and streptavidin as dual FP amplifiers to detect chloramphenicol residues in food. Briefly, label-free aptamer was incubated with chloramphenicol and the aptamer-chloramphenicol conjugate was used as a template. Subsequently, the FAM-labeled forward primer and biotin-labeled reverse primer were added for PCR to amplify the template and the FAM-labeled primer. The molecular weight of FAM-labeled primer increased rapidly and the corresponding FP also enhanced. Finally, with the introduction of streptavidin, the PCR products and streptavidin were combined with the biotin-streptavidin interactions, resulting in much larger molecular weight. Thus, a dual amplified FP signal was obtained. Under optimal conditions, we were able to achieve a wide linear detection range of 0.001–200 nM. In addition, the designed strategy was applied to detect chloramphenicol in honey samples with high accuracy. Moreover, the strategy can be easily extended to detect other small molecules by changing the corresponding aptamers, which provide a promising avenue for the detection of small molecules by FP. Image 1 • A first ever fluorescence polarization aptasensor based on the polymerase chain reaction and streptavidin dual amplification to detect chloramphenicol. • The developed aptasensor was easy to operate with a linear detection range of 0.001–200 nM. • The limit of detection (0.5 pM) was lower than previous work. • This strategy may broaden potential ways to detect other small molecules. [ABSTRACT FROM AUTHOR]