ß-cyclodextrin/spiropyran-functionalized optical-driven hydrogel film for bisphenol A detection in food packaging.

Bisphenol A (BPA), an endocrine disruptor, is widely used in food packaging materials, including drink containers. Sensitive detection of BPA is crucial to food safety. Herein, we have developed a novel optical-driven hydrogel film sensor for sensitive BPA detection based on the displacement of spiropyran (SP) from ß-cyclodextrin (ß-CD) cavity by BPA followed by the photochromism of the released SP. The released SP converts to the ring-opened merocyanine form which shows an enhanced red fluorescence in the dark. The sensor demonstrates a linear detection range from 0.1 to 20 µg mL-1 with a limit of detection at 0.027 µg mL-1 and a limit of quantification at 0.089 µg mL-1. Notably, the proposed ß-CD/SP hydrogel can be reused due to the reversible isomerization of SP and the reversible host-guest interaction. This sensor also shows good performance for BPA determination in real samples, indicating its great potential for food safety monitoring.

Dual-signal detection of tannic acid in red wines based on the peroxidase activity of carbon dots.

Herein, a novel type of phosphorus and iron-doped carbon dot (P,Fe-CD) with outstanding peroxidase activity and excellent fluorescence performance was hydrothermally synthesized to colorimetrically and fluorimetrically detect tannic acid (TA). In the presence of 3,3',5,5'-tetramethylbenzidine (TMB) and H2O2, the P,Fe-CDs could oxidize colorless TMB to a blue oxidation product (oxTMB) resulting in an increased value of absorbance. Simultaneously, the fluorescence intensity of P,Fe-CDs at 430 nm could be quenched owing to the fluorescence resonance energy transfer (FRET) between P,Fe-CDs and the generated oxTMB. Meanwhile, after adding the TA to the system containing TMB, H2O2 and P,Fe-CDs, the value of absorbance could be decreased and the fluorescence could be recovered because of the reduction reaction between TA and oxTMB. Therefore, fluorescence intensity and value of absorbance could be applied to quantitatively detect TA with good linearities between the concentration of TA and the fluorescence intensity/value of absorbance (0.997 and 0.997 for the colorimetric signal and fluorimetric one, respectively) and low limits of detection (0.093 µmol L-1 and 0.053 µmol L-1 for the colorimetry and the fluorimetry, respectively), which was successfully applied to the detection of TA in red wines. Moreover, we applied a smartphone-assisted method to the point-of-care detection of TA with accurate results, providing a new technique for TA detection and food quality monitoring.

A self-cascade system based on Ag nanoparticle/single-walled carbon nanotube nanocomposites as an enzyme mimic for ultrasensitive detection of L-cysteine.

L-Cysteine, widely used in medicine and the food industry, is of great essentiality to organisms and the food quality. Given that current detection approaches require exacting lab conditions and tedious sample treatment, there is a pressing demand for developing a method that possesses advantages of user friendliness, prominent performance, and cost-effectiveness. Herein, a self-cascade system was developed for the fluorescence detection of L-cysteine based on the ingenious performance of Ag nanoparticle/single-walled carbon nanotube nanocomposites (AgNP/SWCNTs) and DNA-templated Ag nanoclusters (DNA-AgNCs). The fluorescence of DNA-AgNCs could be quenched on account of the adsorption of DNA-AgNCs on AgNP/SWCNTs by π-π stacking. With the cooperation of Fe2+, AgNP/SWCNTs with oxidase and peroxidase-like activities could catalyze the oxidation of L-cysteine to produce cystine and hydrogen peroxide (H2O2) and then break the O-O bond of H2O2 to generate a hydroxyl radical (·OH), which could cleave the DNA strand into different sequence fragments which subsequently peeled off from the AgNP/SWCNTs, resulting in a "turn-on" fluorescence response. In this paper, AgNP/SWCNTs with multi-enzyme activities was synthesized enabling the reaction to proceed in just one step. The successful preliminary applications for the L-cysteine detection in pharmaceutical, juice beverage, and serum samples indicated that the developed method exhibited great potential in medical diagnosis, food monitoring, and the biochemical field, which also broadened the horizon for follow-up research.

Competition-Based Two-Dimensional Photonic Crystal Dually Cross-Linked Supramolecular Hydrogel for Colorimetric and Fluorescent Dual-Mode Sensing of Bisphenol A.

Bisphenol A (BPA), one of the most abundantly produced endocrine disrupting chemicals, is widely used in everyday plastic products and thus must be monitored. Multimode sensing platforms are able to combine the advantages of different strategies while solving the issues of inaccurate test results of single signal sensing. However, the exploration in this field is limited due to the compromise of sensing conditions and inevitable mutual interferences of different systems. Herein, we constructed a two-dimensional photonic crystal dually cross-linked supramolecular hydrogel (2DPCDCSH) by utilizing a host-guest pair of ß-cyclodextrin (ß-CD) and tert-butyl (t-Bu) as the second cross-linking for colorimetric and fluorescent dual-mode sensing of BPA. Based on the fact that BPA can act as a competitive guest to break the host-guest interaction between ß-CD and t-Bu, the cross-linking density decreased and an expansion-induced structural color change occurred. Sensitive and selective BPA detection can be easily achieved by measuring the Debye diffraction ring diameter or observing the color change of 2DPC with a detection limit of 1 µg mL-1. Moreover, the formation of the ß-CD/BPA complex gave a significant enhancement of the intrinsic fluorescence of BPA, obtaining a detection limit of 0.001 µg mL-1. The two sensing systems can share the same reaction condition and yield a wider dynamic response range than the single signal strategy. Overall, the proposed method presented an efficient, rapid, cost-effective, and regenerative dual-mode method for BPA analysis and shed new insights for the design of diversified sensing platforms.

A ratiometric fluorescence strategy based on copper nanoclusters/carbon dots for sensitive detection of doxorubicin.

Sensitive detection of doxorubicin (DOX) is critical for clinical theranostics. A novel ratiometric fluorescence strategy based on the inner filter effect (IFE) has been established for the sensitive detection of DOX by designing a ratiometric fluorescence probe. In the presence of DOX, the fluorescence intensity of copper nanoclusters (CuNCs) at 485 nm decreases, and the fluorescence intensity of carbon dots at 560 nm increases. Therefore, DOX can be quantitatively detected by measuring the ratio of the fluorescence intensities at 560 and 485 nm (F560 /F485 ). The F560 /F485 ratio exhibits a linear correlation with the DOX concentration in the range from 1.0 × 10-8 M to 1.0 × 10-4 M with the detection limit of 3.7 nM. Furthermore, this method was also successfully applied to the analysis of DOX in human plasma samples, affording an effective platform for drug safety management.

Construction and Research of Multiple Stimuli-Responsive 2D Photonic Crystal DNA Hydrogel Sensing Platform with Double-Network Structure and Signal Self-Expression.

The stimuli-responsive DNA hydrogel has attracted wide attention in the fields of chemical and biological sensing. However, it is still a challenge to integrate characteristics with low-cost, high mechanical strength, and signal self-expression into a DNA hydrogel simultaneously. Herein, a stimuli-responsive 2D photonic crystal double network DNA hydrogel (2D PhC DN-DNA hydrogel) sensing platform is developed via combining the signal self-expression of 2D PhC array with the selective recognition of polyacrylamide (PAM)/DNA DN hydrogel. The change of DNA configuration induced by specific target triggers the change of 2D PhC DN-DNA hydrogel volume, leading to a shift of the Debye diffraction ring diameter. In order to verify the feasibility of this strategy, the 2D PhC DN-DNA hydrogel with C-rich sequences is chosen as a proof-of-concept. The results indicate that the hydrogel has good detection performance for pH and Ag+/Cys. And the Debye diffraction ring diameter of the hydrogel is correlated with the concentration of the Ag+/Cys in the range of 0.5-20 µM. Compared with previously pure DNA hydrogel sensing platform, the 2D PhC DN-DNA hydrogel features low-cost preparation process and label-free determination. Meanwhile, only a laser pointer and a ruler are needed for the determination of targets, which shows that the hydrogel has application prospect in the development of portable response equipment.

A novel liquid crystal sensing platform for highly selective UO22+ detection based on a UO22+-specific DNAzyme.

A label-free and selective sensor was established for uranyl ion (UO22+) detection based on a UO22+-dependent DNAzyme and liquid crystals (LCs). In the presence of UO22+, the substrate chains can be cleaved at the rA site by the DNAzyme strands. The cleaved products released from the DNAzyme strand will hybridize with the capture probes that are fixed on the LC sensing substrate to form double strands. The formation of double strands would disturb the original orientation and induce the rearrangement of liquid crystal molecules, resulting in the polarization images changing from uniform black to bright. Attributed to the specificity of the DNAzyme and the optical signal of the LC, a highly selective and label-free method was established with a detection limit of 25 nM. This approach showed satisfactory analytical performance and offered an inspiring platform for detecting other radioactive elements.

Control of Liquid Crystal Microarray Optical Signals Using a Microspectral Mode Based on Photonic Crystal Structures.

Herein, a novel liquid crystal microarray (LCM) film with optical regulation ability is first constructed by combining liquid crystals (LC) and the highly ordered microporous structure of inverse opal photonic crystals (IOPhCs). The LCM films are fabricated by infiltrating LC molecules into the LC polymer with the structure of IOPhCs, and their properties are very different from those without the LC. Interestingly, the optical property of LCM films can be controlled by changing the orientation of LC molecules, which varies with the interfacial force. In combination with polarization images, spectral reflection peak, circular dichroism spectra, potential difference, and fluorescence images of LCM films, the mechanism of this change is investigated. It is found that the exposed basic group of single-stranded DNA is the key to the change of the optical property of LC microarrays. Meanwhile, the optical signals of LC microarrays based on the PhCs provide a novel LC signal mode for an LC sensing system (microspectral signal mode), and it can be recorded by a fiber-optic spectrometer, which is a great improvement on LC sensing signals. Therefore, the LC microarray sensing signal can be used for accurate analysis of targets by the change of the reflection peak intensity of PhCs. When the LC molecules are induced by different aptamers, the LC microarray sensing interface can be further used for the determination of different targets, such as cocaine and Hg2+. The research on LCM films is of significant value for the development of LC sensing technology and also shows great application prospects in biochemical sensing fields.

A two-dimensional photonic crystal hydrogel biosensor for colorimetric detection of penicillin G and penicillinase inhibitors.

A simple penicillinase functionalized two-dimensional photonic crystal hydrogel (2DPPCH) biosensor was developed for colorimetric detection of penicillin G and penicillinase inhibitors. The penicillinase can specifically recognize penicillin G and catalyze it to produce penicilloic acid, which decreases the pH of the hydrogel microenvironment and shrinks the pH-sensitive hydrogel. The particle spacing decrease of the 2D photonic crystal array induced by the hydrogel shrinkage further causes a blue-shift in the diffraction wavelength. While the hydrolysis reaction is repressed upon treatment with clavulanate potassium (a kind of penicillinase inhibitor), no significant change in the diffraction wavelength is found. The detection of targets can be achieved by measuring the Debye diffraction ring diameter or observing the structural color change in the visible region. The lowest detectable concentrations for penicillin G and clavulanate potassium are 1 µM and 0.1 µM, respectively. Moreover, the 2DPPCH is proved to exhibit high selectivity and an excellent regeneration property, and it shows satisfactory performance for penicillin G analysis in real water samples.

Dual-Signal Amplification Strategy for Sensitive MicroRNA Detection Based on Rolling Circle Amplification and Enzymatic Repairing Amplification.

MicroRNAs (miRNAs) play crucial regulatory roles as post-transcriptional regulators for gene expression and serve as promising biomarkers for diagnosis and prognosis of diseases. Herein, a dual-signal amplification method has been developed for sensitive and selective detection of miRNA based on rolling circle amplification (RCA) and enzymatic repairing amplification (ERA) with low nonspecific background. This strategy designs a padlock probe that can be cyclized in the presence of target miRNA to initiate the RCA reaction, after which the TaqMan probes that are complementary to the RCA products can be cyclically cleaved to produce obvious fluorescence signals with the help of endonuclease IV (Endo IV). Attributed to the dual-signal amplification procedure and the high fidelity of Endo IV, the RCA-ERA method allows quantitative detection of miR-21 in a dynamic range from 2 pM to 5 nM with a low background signal. Moreover, it has the ability to discriminate single-base difference between miRNAs and shows good performance for miRNA detection in complex biological samples. The results demonstrate that the RCA-ERA assay holds a great promise for miRNA-based diagnostics.

Colorimetric detection uranyl ions based on the enhanced peroxidase-like activity by GO adsorption.

Based on the fact that uranyl ions (UO22+) adsorbed on GO can enhanced the peroxidase-like activity of graphene oxide (GO), a novel colorimetric strategy for visualizing quantitative determination of uranyl ions was established. The peroxidase-like activity of GO-UO22+ nanocomposites was assessed by catalyzing H2O2 oxidation of TMB to produce a distinct color reaction. A good linearity between the UO22+ concentration and absorption at 652 nm was acquired in the range of 5.90 × 10-6 to 9.43 × 10-4 M with a detection limit of 4.70 µM. This strategy was also successfully applied to determination of uranyl ions in environmental water samples.

Amplified electrochemical determination of UO22+ based on the cleavage of the DNAzyme and DNA-modified gold nanoparticle network structure.

A superior electrochemical biosensor was designed for the determination of UO22+ in aqueous solution by integration of DNAzyme and DNA-modified gold nanoparticle (DNA-AuNP) network structure. Key features of this method include UO22+ inducing the cleavage of the DNAzyme and signal amplification of DNA-AuNP network structure. In this electrochemical method, the DNA-AuNP network structure can be effectively modified on the surface of gold electrode and then employed as an ideal signal amplification unit to generate amplified electrochemical response by inserting a large amount of electrochemically active indicator methylene blue (MB). In the presence of UO22+, the specific sites on DNA-AuNP network structure can be cleaved by UO22+, releasing the DNA-AuNP network structure with detectable reduction of electrochemical response intensity. The electrochemical response intensity is related to the concentration of UO22+. The logarithm of electrochemical response intensity and UO22+ concentration showed a wide linear range of 10~100 pM, and the detection limit reached 8.1 pM (S/N = 3). This method is successfully used for determination of UO22+ in water samples. Graphical abstract Fabricated DNAzyme network structure for enhanced electrical signal. Numerical experiments show that the current signal decreases as the concentration of UO22+ increases. It can be seen that the biosensors could be used to detect UO22+ in aqueous solution effectively.

Colorimetric strategy for ascorbic acid detection based on the oxidase-like activity of silver nanoparticle single-walled carbon nanotube composites.

A colorimetric assay for the determination and quantification of ascorbic acid (AA) is presented using silver nanoparticle (AgNP) single-walled carbon nanotube (AgNP/SWCNT) nanocomposites prepared using a microwave-assisted method. The AgNP/SWCNT nanocomposites possessed oxidase-like properties toward 3,3',5,5'-tetramethylbenzidine (TMB) and could catalyze the oxidation of TMB to form a blue oxidation product (λmax = 652 nm) in the absence of H2 O2 . AA can specifically inhibit the oxidation of TMB, resulting in a decline of the absorbance value and blue colour fading. As such, amounts of AA can be assessed easily by the unaided eye and quantitatively using an ultraviolet-visible light spectrophotometer. Under the optimal reaction conditions, this strategy showed a good linearity ranging from 0.4 µM to 5.0 µM for AA detection, and the limit of detection was 130 nM. This assay was also applied for AA measurement in vitamin C tablets and juice samples that yielded satisfactory results.

A cytosine-rich hairpin DNA loaded with silver nanoclusters as a fluorescent probe for uranium(IV) and mercury(II) ions.

A dually responsive fluorescent probe for determination of U(IV) and mercury(II) ions was synthesized. The probe consists of a cytosine-rich hairpin DNA loaded with silver nanoclusters (DNA-AgNCs). The fluorescence of the AgNCs is found to be quenched by UO2(II) at pH 5.0 and Hg(II) at pH 7.0 due to combined static and dynamic quenching. Under the optimal conditions, the green fluorescence of the DNA-AgNCs, best measured at excitation/emission wavelengths of 420/525 nm, decreases in the 4.0 to 75 pM UO2(II) concentration range, and in the 0.3 to 8.0 nM Hg(II) concentration range. The respective detection limits are as low as 1.8 pM and 0.1 nM. The method was successfully applied to the determination of UO2(II) and Hg(II) in (spiked) pond and taps waters and in soil extracts. Graphical abstract A label-free DNA was designed to synthesize green-fluorescent silver nanoclusters (AgNCs) and used for rapid dual detection of uranyl ions (UO2(II)) at pH 5.0 and of mercury ions (Hg(II)) at pH 7.0 in environmental samples.

A label-free and highly sensitive strategy for uracil-DNA glycosylase activity detection based on stem-loop primer-mediated exponential amplification (SPEA).

Uracil-DNA glycosylase (UDG) plays essential roles in base excision repair (BER) pathway by eliminating uracil from DNA to sustain the genome integrity. Sensitive detection of UDG activity is of great significance in the study of many fundamental biochemical processes and clinical applications. We develop a label-free method for UDG activity detection using stem-loop primer-mediated exponential amplification (SPEA). In the presence of active UDG, the uracil base in helper hairpin probe (HP) can be excised to generate an abasic site (AP site), which can be cleaved by endonuclease IV (Endo IV) with a blocked primer released. This primer then triggers the strand displacement reaction to produce a dumb-bell structure DNA, which can initiate a loop-mediated isothermal amplification (LAMP) reaction. This reaction generates a large number of long double-strand DNA replicates, which can be stained by SYBR Green (SG) I to deliver enhanced fluorescence for quantitative detection of UDG activity. A linear range from 0.001 U/mL to 1 U/mL and a detection limit down to 0.00068 U/mL are achieved. This strategy has also been demonstrated for UDG assay in complex cell lysates, implying its great potential for UDG based clinical diagnostics and therapeutics.

Novel Aptasensor Platform Based on Ratiometric Surface-Enhanced Raman Spectroscopy.

A novel aptasensor platform has been developed for quantitative detection of adenosine triphosphate (ATP) based on a ratiometric surface-enhanced Raman scattering (SERS) strategy. The thiolated 3'-Rox-labeled complementary DNA (cDNA) is first immobilized on the gold nanoparticle (AuNP) surface and then hybridizes with the 3'-Cy5-labeled ATP-binding aptamer probe (Cy5-aptamer) to form a rigid double-stranded DNA (dsDNA), in which the Cy5 and Rox Raman labels are used to produce the ratiometric Raman signals. In the presence of ATP, the Cy5-aptamer is triggered the switching of aptamer to form the aptamer-ATP complex, leading to the dissociation of dsDNA, and the cDNA is then formed a hairpin structure. As a result, the Rox labels are close to the AuNP surface while the Cy5 labels are away from. Therefore, the intensity of SERS signal from Rox labels increases while that from Cy5 labels decreases. The results show that the ratio between the Raman intensities of Rox labels and Cy5 labels is well linear with the ATP concentrations in the range from 0.1 to 100 nM, and the limit of detection reaches 20 pM, which is much lower than that of other methods for ATP detection and is also lower than that of SERS aptasensor for ATP detection. The proposed strategy provides a new reliable platform for the construction of SERS biosensing methods and has great potential to be a general method for other aptamer systems.

Sensitive inkjet printing paper-based colormetric strips for acetylcholinesterase inhibitors with indoxyl acetate substrate.

A new paper-based biosensing approach has been developed for sensitive and rapid detection of acetylcholinesterase (AChE) inhibitors. The biosensing zone of the paper strip is constructed with an inkjet printing method, and the biomolecule AChE is immobilized into two layers of biocompatible sol-gel-derived silica ink with a "sandwich" form. Indoxyl acetate (IDA) is used as a chromogenic substrate, which is colorless and can be catalytically hydrolyzed into blue-colored indigo dipolymer. When the enzymatic activity of AChE is inhibited after incubation with organophosphate pesticides (OPs), there is a decreased hydrolysis of IDA accompanying with a drop in color intensity. Paraoxon and trichlorfon are used as the representative OPs in the assay. Due to the low solubility and high molar absorption coefficient of the IDA dipolymer product, the paper-based strip can form a neat blue sensing zone and shows obviously improved sensitivity with a limit of detection (LOD) of 0.01ngmL-1 paraoxon and 0.04ngmL-1 trichlorfon (S/N=3) and the LODs for visual detection are 0.03ngmL-1 for paraoxon and 0.1ngmL-1 for trichlorfon comparing with the previously reported colorimetric methods. The concentrations of paraoxon in apple juice samples are also detected, and the results are in accord well with these results from high-performance liquid chromatography, showing great potential for on-site detection of OPs in practical application. The developed assay can be used to qualitatively and semiquantitatively estimate with naked eyes and quantitatively assess OPs through image analysis.

Label-Free Photonic Crystal-Based ß-Lactamase Biosensor for ß-Lactam Antibiotic and ß-Lactamase Inhibitor.

A simple, label-free, and visual photonic crystal-based ß-lactamase biosensor was developed for ß-lactam antibiotic and ß-lactamase inhibitor in which the penicillinase (a ß-lactamase) was immobilized on the pH-sensitive colloidal crystal hydrogel (CCH) film to form penicillinase colloidal crystal hydrogel (PCCH) biosensing film. The hydrolysis of penicillin G (a ß-lactam antibiotic) can be catalyzed by penicillinase to produce penicilloic acid, leading to a pH decrease in the microenvironment of PCCH film, which causes the shrink of pH-sensitive CCH film and triggers a blue-shift of the diffraction wavelength. Upon the addition of ß-lactamase inhibitor, the hydrolysis reaction is suppressed and no clear blue-shift is observed. The concentrations of ß-lactam antibiotic and ß-lactamase inhibitor can be sensitively evaluated by measuring the diffraction shifts. The minimum detectable concentrations for penicillin G and clavulanate potassium (a ß-lactamase inhibitor) can reach 1 and 0.1 µM, respectively. Furthermore, the proposed method is highly reversible and selective, and it allows determination of penicillin G in fish pond water samples.

A novel logic gate based on liquid-crystals responding to the DNA conformational transition.

Described herein is a novel liquid crystal (LC)-based DNA logic gate constructed via employing the reorientation of LCs triggered by metal-ion-mediated DNA probe conformational changes.