Phosphorescent immunosensor for simple and sensitive detection of microcystin-LR in water

A simple and sensitive Mn–ZnS quantum dot room-temperature phosphorescent immunosensor for detecting microcystin-LR was developed. This sensor adopted antigens and antibodies as recognition units and used Mn–ZnS RTP QDs as sensing materials to specifically bind with MC-LR. The structurally specific binding between the microcystin-LR antibody and MC-LR led to the aggregation of antibody-crosslinked QDs, and then the electrons of QDs would be transferred to the complex, leading to the phosphorescence quenching of QDs. The microcystin-LR antigen–antibody specific binding site was first analyzed. This phosphorescent immunosensor rapidly and sensitively detected microcystin-LR, with linear ranges of 0.2–1.5 μg L⁻¹ and 1.5–20 μg L⁻¹ and a detection limit of up to 0.024 μg L⁻¹. Meanwhile, coexisting pollutants of microcystin-LR in water did not significantly interfere with microcystin-LR detection. The new sensor was applied to detect real water samples and showed high sensitivity and selectivity.

A simple and sensitive Mn–ZnS quantum dot room-temperature phosphorescent immunosensor for detecting microcystin-LR was developed.     

Dual-emissive ratiometric fluorescent nanosensor based on multi-nanomaterials for Ag+ determination in lake water

In this study, a sensitive ratiometric fluorescent nanosensor was constructed using a facile one-pot method by encapsulating carbon dots (CDs) and cadmium telluride quantum dots (CdTe QDs) into the pore cavities of a metal–organic framework (ZIF-8). In this nanosensor (CD/CdTe QD@ZIF-8), the fluorescence attributed to CdTe QDs was quenched by silver ions (Ag⁺), and the fluorescence intensity of CDs did not change. The introduction of ZIF-8 into the system can not only adsorb Ag⁺ but also easily separate CDs and CdTe QDs from the matrix. The developed CD/CdTe QD@ZIF-8 composite used as a ratiometric fluorescent probe exhibited high sensitivity and selectivity towards Ag⁺. The working linear range was 0.1–20 μM with a limit of detection (LOD) of 1.49 nM. Finally, the proposed nanosensor was applied to determine Ag⁺ in lake water with satisfactory results.

A novel ratiometric fluorescent sensor (CDs/CdTe QDs@ZIF-8) combining the advantages of ratiometric and MOFs was synthesized for Ag⁺ detection.     

Silver ion-doped CdTe quantum dots as fluorescent probe for Hg2+ detection

Mercury(ii), which is a well-known toxic species, exists in the industrial waste water in many cases. In the present work, CdTe quantum dots (QDs) are studied as a fluorescence probe for Hg²⁺ detection. Ag ions are induced to QDs to enlarge their detection concentration range. l-cysteine is employed in the QD-based fluorescence probe to connect QDs with Hg²⁺. X-ray diffraction, transmission electron microscopy and X-ray photoelectron spectroscopy results indicate the formation of zinc blende CdTe QDs with sizes of ∼5 nm and the existence of Ag⁺ in crystalline CdTe. Photoluminescence (PL) spectra and PL decay spectra were acquired to investigate the emission mechanism of Ag-doped CdTe QDs, revealing multi-emission in QD samples with higher concentrations of Ag⁺ doping. The highest PL quantum yield of the QD samples was 59.4%. Furthermore, the relationship between the fluorescence intensity and the concentration of Hg²⁺ has been established. Two linear relationships were obtained for the plot of F/F₀ against Hg²⁺ concentration, enlarging the detection concentration range of Hg²⁺.

Ag-doped CdTe QDs emit multiple-fluorescence peaks, and the relationship between fluorescence intensity and the concentration of Hg²⁺ is established. Two linear relationships are obtained, which is benefit to the extension of detection range.     

Microfluidic fluorescent platform for rapid and visual detection of veterinary drugs

The overuse of veterinary drugs and veterinary drug residues is increasingly becoming an obstacle to sustainable development worldwide. It is therefore imperative to establish a quantitative, sensitive and efficient method for the detection of veterinary drugs. Herein, we developed a visual microfluidic detection platform for rapid and sensitive detection of veterinary drugs using CdTe quantum dots (QDs) with three different ligands as the sensing units. Green-emissive 3-mercaptopropionic acid (MPA)-CdTe QDs, yellow-emissive thioglycolic acid (TGA)-CdTe QDs and orange-emissive N-acetyl-l-cysteine (NAC)-CdTe QDs were synthesized by a sulfhydryl aqueous phase method. These CdTe QDs show selective rapid fluorescence response to pefloxacin (PEF), malachite green (MG), and 1-aminohydantoin hydrochloride (AHD). With the concentration of veterinary drugs increasing, the CdTe QDs reveals a fluorescence color variation from bright to dark until quenched and the response degree of CdTe QDs with different ligands to veterinary drugs is different. Specifically, the limits of detection (LODs) of MPA-CdTe, TGA-CdTe and NAC-CdTe QDs probes for PEF were 7.57 μM, 1.75 μM and 2.90 μM, respectively, and the response was complete in a few seconds, realizing the sensitive and rapid detection of PEF. The three kinds of CdTe QDs could also be used in the detection of other veterinary drugs such as MG and AHD. Finally, a microfluidic detection platform was constructed for visual sensing and rapid detection towards veterinary drugs. The sensor platform holds the advantages of simple operation, low cost, rapid sensing and good sensitivity, and is potentially useful for visual quantitative detection of veterinary drug residues in aquatic products and the environment.

A visual microfluidic fluorescent detection platform based on multicolor quantum dots with multiple capping ligands is developed for rapid and sensitive detection of veterinary drugs.     

A fluorescent microsensor for the selective detection of bifenthrin

Based on the fluorescence quenching phenomenon, a smart fluorescent microsensor was synthesized. The bifenthrin (BI) microsensor inherited the high selectivity of molecular imprinted polymers (MIPs) and the excellent fluorescence properties of aqueous CdTe quantum dots (QDs). Aqueous CdTe QDs are functionalized by octadecyl-4-vinylbenzyl-dimethyl-ammonium chloride (OVDAC). A type of functional monomer, 4-vinylphenylboronic acid (VPBA), was used and its boronic acid groups could covalently combine with a cis-diol compound for direct imprinting polymerization. The OVDAC-functionalized aqueous CdTe QDs were used as solid supports and auxiliary monomers. Under optimal conditions, experimentation showed that BI had a linear detection range of 10 to 300 μmol L⁻¹ with a correlation coefficient of 0.9968 and a high imprinting factor (IF) of 4.53. In addition, the prepared MIP-OVDAC/CdTe QDs were successfully used to detect BI in water samples. Therefore, this work provided a highly selective and sensitive fluorescence probe for the detection of BI. In addition, the fluorescence probe could be used to detect other targets by changing the functional monomers.

Based on the fluorescence quenching phenomenon, a smart fluorescent microsensor was synthesized.     

Preparation of CdTe superparticles for white light-emitting diodes without Förster resonance energy transfer

Due to many unique and excellent optical properties, quantum dots (QDs) have been seen as one of the most promising color conversion materials in light-emitting diodes (LEDs). However, the Förster resonance energy transfer (FRET) among different colored QDs always causes a significant red-shift of the fluorescence emission, impeding the fabrication of LEDs with predicted photoluminescence (PL) emission spectra. In this work, we take advantage of CdTe superparticles (SPs), which are assembled by CdTe QDs, as the color conversion materials for the fabrication of WLEDs. Because of their submicron size, the distance between QDs with different emissions can be large enough to avoid the FRET process. More importantly, this method provides us with an opportunity to precisely design and regulate the PL emission spectra of LEDs. By easily overlapping the individual PL spectra of CdTe SPs with different emissions, the certain ratio of their usage for fabricating LEDs with desired PL emission spectra is identified. According to this idea, a WLED with a color rendering index (CRI) of 81, luminous efficacy of 27 lm W⁻¹, and color coordinate at (0.33, 0.34) with the color temperature of 5742 K is achieved.

CdTe superparticles are used as the color conversion materials for the fabrication of WLEDs without FRET.     

Development of a FRET-based fluorescence aptasensor for the detection of aflatoxin B1 in contaminated food grain samples

The present study aimed to develop an aptamer-based FRET detection strategy for the specific and sensitive detection of AFB1 in contaminated food grains. The study comprises generation of ssDNA aptamers against AFB1 by whole-cell SELEX and their application in a FRET-based platform utilizing graphene oxide (GO) and quantum dots (QDs). The generated aptamers were characterized to determine their specificity and sensitivity using indirect ELISA where AFB1–OVA was used as a coating antigen. Among the aptamers generated, the ATB1 aptamer showed good reactivity and selectivity against AFB1. This aptamer was further characterized to determine its secondary structure and KD value, which was found to be 5.9 kcal mol⁻¹. The characterized aptamers were conjugated onto Cd/Se quantum dots to develop a fluorimetric system for the detection of aflatoxin B1 using a graphene oxide platform. The presence of graphene oxide quenches the fluorescence ability of the quantum dots due to π–π stacking interactions between the aptamer and GO. Upon target addition, the aptamer forms a complex with aflatoxin B1 thereby restoring the fluorescence intensity. The developed assay shows a linear response from 0.002 μg μl⁻¹ to 0.2 μg μl⁻¹ with a detection limit of 0.004 μg μl⁻¹ for the AFB1 standard toxin and showed no cross-reactivity with other closely related mycotoxins. To validate the reliability of the developed method, several field samples spiked with AFB1 were included in this study and the results obtained were cross verified using a standard commercial AFB1 kit. In conclusion, the developed method may find good utility in routine food testing laboratories for risk assessment of AFB1.

The present study aimed to develop an aptamer-based FRET detection strategy for the specific and sensitive detection of AFB1 in contaminated food grains.     

The design of Mn2+&Co2+ co-doped CdTe quantum dot sensitized solar cells with much higher efficiency

High quality Mn²⁺-doped CdTe quantum dots (QDs), Co²⁺-doped CdTe QDs and Mn²⁺&Co²⁺ co-doped CdTe QDs were successfully synthesized via an aqueous phase method with mercaptopropanoic acid (MPA) ligands. The doped QDs maintain the same zinc blende structure of CdTe by X-ray diffraction (XRD). The Mn²⁺-doped CdTe QDs and Co²⁺-doped CdTe QDs both show a red-shift on absorption and photoluminescence (PL) spectra compared to pure CdTe QDs. In addition, Mn²⁺-doped CdTe QDs show a significant increase in the PL lifetime due to an orbitally forbidden d–d transition, which is of benefit to the reduction of electron recombination loss. Co²⁺ doping has a more matched doping energy level. In view of this, Mn²⁺&Co²⁺ co-doped CdTe QDs were applied as sensitizers for quantum dot sensitized solar cells, resulting in a significantly enhanced efficiency.

Mn²⁺&Co²⁺ co-doped CdTe QDs were successfully synthesized via aqueous phase method with mercaptopropanoic acid, which could generate different doping energy levels, and as sensitizers applied for QDSCs, showing a significantly enhanced efficiency.     

Highly sensitive cadmium sulphide quantum dots as a fluorescent probe for estimation of doripenem in real human plasma: application to pharmacokinetic study

Thioglycolic acid-capped cadmium sulphide quantum dots (TGA-CdS QDs) have been synthesized and utilized as a fluorescent probe for the estimation of doripenem (DOR). Monitoring of DOR in different biological fluids is required to estimate the efficient dose to avoid bacterial infections and resistance. The investigated method is based on the measurement of fluorescence quenching of TGA-CdS QDs after the addition of DOR. The synthesized TGA-CdS QDs were characterized using transmission electron microscopy (TEM), Fourier transform infrared (FTIR) spectroscopy, X-ray powder diffraction (XRD) and ZETA sizer. The TGA-CdS QDs showed unique photophysical properties with high quantum yield (0.32) using a comparison method with rhodamine B. Different experimental parameters affecting the synthesis process of the TGA-CdS QDs and their behavior with the studied drug DOR were examined and optimized. The values of the fluorescence quenching were linearly correlated to DOR concentration over the range of 10–500 ng mL⁻¹ with a good correlation coefficient of 0.9991. The proposed method showed higher sensitivity over several reported methods, with LOD reaching 2.0 ng mL⁻¹. The method was effectively applied for the estimation of DOR in human plasma and urine with good recovery results ranged from 95.16% to 99.51%. Furthermore, the stability of DOR in the human plasma was studied and a pharmacokinetic study of DOR in real human plasma was conducted.

Thioglycolic acid-capped cadmium sulphide quantum dots (TGA-CdS QDs) as a fluorescent sensor have been synthesized and utilized as a fluorescent probe for the estimation of doripenem (DOR) in human plasma.     

CdTe QD-based inhibition and reactivation assay of acetylcholinesterase for the detection of organophosphorus pesticides

An enzyme immobilized glutathione (GSH)-capped CdTe quantum dot (QD)-based fluorescence assay has been developed for monitoring organophosphate pesticides. In principle, GSH-capped CdTe QDs exhibit higher sensitivity towards H₂O₂ produced from the active enzymatic reaction of acetylcholinesterase (AChE) and choline oxidase (CHOx), which results in the fluorescence (FL) “turn-off” of the GSH-capped CdTe QDs. A “turn-on” FL of the CdTe QDs at 520 nm was recovered in the presence of organophosphate (OP). The FL changes of the GSH-capped CdTe QD/AChE/CHOx biosensor reasonably correspond to the amount of OP pesticides. The detection limit of the CdTe/AChE/CHOx biosensor towards paraoxon, dichlorvos, malathion and triazophos was 1.62 × 10⁻¹⁵ M, 75.3 × 10⁻¹⁵ M, 0.23 × 10⁻⁹ M and 10.6 × 10⁻¹² M, respectively. The GSH-capped CdTe QDs/AChE/CHOx biosensor was applied as a FL nanoprobe for assaying the enzymatic activity of AChE. The inhibited AChE was reactivated up to 94% using pyridine oximate (2-PyOx⁻), and functionalized pyridinium oximates (4-C₁₂PyOx⁻ and 4-C₁₈PyOx⁻) of varying chain lengths. It was found that the reactivation potency of the tested oximes varied with the chain length of the oximes. This biosensing system offers the promising benefit for the determination of the OP pesticides in food, water and environmental samples.

An enzyme immobilized glutathione (GSH)-capped CdTe quantum dot (QD)-based fluorescence assay has been developed for monitoring organophosphate pesticides.     

A fluorescence immunoassay based on CdTe : Zn/ZnS quantum dots for the rapid detection of bacteria,taking Delftia tsuruhatensis CM’13 as an example

A fluorescence immunoassay has been widely applied in different fields due to its high sensitivity, simple operations, and high accuracy. Quantum dots (QDs) are often selected as labels in a fluorescence immunoassay due to their high fluorescence, better stability, and biocompatibility. In this study, novel doped CdTe : Zn/ZnS QDs with stability and a high photoluminescence quantum yield (40.78%) were prepared by the water synthesis method and used as labels to conjugate with goat anti-rabbit IgG to establish a fluorescence immunoassay (FLISA) for bacteria compared to the traditional enzyme-linked immunosorbent assay (ELISA) based on the reaction between an antibody and an antigen. A good linear relationship between the fluorescence intensity and concentrations of D. tsuruhatensis CM13 was found when the concentrations were in the range of 10³ CFU mL⁻¹–10⁸ CFU mL⁻¹. The limit of detection (LOD) of D. tsuruhatensis CM13 was 1.25 × 10³ CFU mL⁻¹ by FLISA, which was about 80 times lower than the LOD obtained from ELISA (10⁵ CFU mL⁻¹). This indicated that our FLISA method has higher sensitivity than traditional ELISA, and the CdTe : Zn/ZnS QDs synthesized in this paper have good applications in the rapid sensitive detection of microorganisms.

A fluorescence immunoassay has been widely applied in different fields due to its high sensitivity, simple operations, and high accuracy.     

Simple strategy for sensitive detection of dopamine using CdTe QDs modified glassy carbon electrode

Background

Cellular and brain metabolism of dopamine can be correlated with a number of neurodegenerative disorders, our study was to explore a simple and efficient method to detect dopamine in real samples.

Methods

A new quantum dots (CdTe QDs) could be prepared using the hydrothermal method, the electrochemical biosensor was established by dropping CdTe QDs on the surface of glassy carbon electrode (GCE).

Results

The CdTe QDs/GCE exhibited the excellent electrochemical catalytic activity toward dopamine (DA) with good stability and high sensitivity in presence of interfering substances. The detection limit of DA was calculated by differential pulse voltammetry (DPV) as low as 0.3 μmol L⁻¹ with a linear dynamic range of 1 μmol L⁻¹ to 400 μmol L⁻¹.

Conclusion

In this paper, the proposed electrochemical biosensor could be effectively used for the direct and rapid detection of DA in human serum and urine samples.

     

An approach toward miRNA detection via different thermo-responsive aggregation/disaggregation of CdTe quantum dots

MicroRNA-155 regulates the expression of 147 target genes that are involved in cancer pathways, and its expression level has been shown to be up-regulated in breast cancer. Thus, it is necessary to investigate the value of microRNA-155 for early diagnosis and prognosis of breast cancer. Here we present a novel and “light shift” spectral method for the detection of miRNA based on different thermo-responsive aggregation/disaggregation of CdTe quantum dots (CdTe QDs) by using single stranded DNA or a DNA/RNA heteroduplex as a template after heat treatment. In this method upon addition of the DNA/RNA heteroduplex, the CdTe QDs aggregate strongly due to their strong interaction with the double stranded nucleic acid, which results in fluorescence quenching. By applying the melting temperature (T_m), the DNA/RNA heteroduplex denatures and two strands are dissociated, which disaggregates the QDs, effectively switching to fluorescence emission of QDs. These processes were investigated with Atomic Force Microscopy (AFM) and fluorescence spectroscopy. The proposed method has been used also for the determination of miR-155 in total RNAs extracted from the human breast carcinoma SK-BR-3 cells and normal human embryonic kidney cell line (HEK 293).

A novel and “light shift” spectral method for the detection of miRNA based on different thermal-responsive aggregation/disaggregation of CdTe quantum dots was investigated.     

A fluorescence sensing platform of theophylline based on the interaction of RNA aptamer with graphene oxide

RNA, with a structure similar to DNA, should exhibit similar behaviors when it interacts with graphene. In this work, we designed a sensing platform of theophylline based on the interaction of an RNA aptamer with graphene oxide (GO) using the fluorescence as a sensing signal. Firstly, quantum dots (QDs) were modified with the selected ssRNA that can be used as an aptamer to recognize the theophylline. The fluorescence of QDs will be quenched in the presence of GO due to the noncovalent assembly between ssRNA aptamer and GO, leading to fluorescence resonance energy transfer (FRET) from QDs to GO, fluorescence “turn-off”. Then, in the presence of theophylline, the ssRNA aptamer recognizes theophylline to form a dsRNA–theophylline complex. The weak affinity between the complex and GO makes QDs move away from the GO surface, leading to the fluorescence recovery of QDs, fluorescence “turn-on”. Because of the high fluorescence quenching efficiency, unique structure of GO and specificity of the RNA aptamer, the proposed sensing platform exhibits high sensitivity and excellent selectivity for the determination of theophylline. The excellent performance of the sensor based on GO provides new opportunities for sensitive and selective detection of biorecognition events.

A fluorescent sensing platform of theophylline based on the interaction of an RNA aptamer with GO and CdTe as the signal.     

Detection of nucleic acids via G-quadruplex-controlled l-cysteine oxidation and catalyzed hairpin assembly-assisted signal amplification

The development of simple, sensitive and cost-effective methods for specific nucleic acid detection has attracted tremendous attention due to its importance to the early diagnosis of genetic diseases and to biodefense applications. In this work, we demonstrated a fluorescent turn-off mode DNA assay based on l-cysteine-modulated synthesis of CdTe quantum dots (CdTe QDs), horseradish peroxidase-mimicking G-quadruplex–hemin–K⁺ complex controlled oxidation of l-cysteine to cystine, and catalyzed hairpin assembly (CHA)-assisted signal amplification. After the addition of target DNA, the CHA signal amplification reaction was triggered and numerous H1–H2 double-stranded DNA were formed, initiating the release of G-quadruplex sequences in H2 simultaneously. Thus, the degree of inhibition of the synthesis of CdTe QDs is proportional to the concentration of the G-quadruplex sequence in this method. In contrast, when the target DNA was absent, the CHA could not be triggered, and the fluorescence signal was high due to the remaining intact l-cysteine. Under optimal experimental conditions, the homogeneous fluorescence method achieved the detection of HIV DNA with a linear range from 0.1 pM to 1 nM and a detection limit of 0.12 pM. This novel biosensor exhibits excellent specificity in differentiating DNA sequences with a single-base and two-base mismatch. To the best of our knowledge, this a label-free and highly sensitive bioassay utilizing CHA-assisted signal amplification and G-quadruplex control of in situ synthesis of CdTe QDs strategy was not reported in previous. Thus, this proposed strategy is anticipated to find use in basic biochemical research and clinical diagnosis.

A novel homogeneous strategy for detection of DNA via biomimetic synthesis of luminescent QDs coupled with nucleic acid signal amplification.     

Ultra-highly fluorescent N doped carbon dots-CdTe QDs nanohybrids with excitation-independent emission in the blue-violet region

Luminescent carbon dots (CDs) are of significant practical application interest such as in optoelectronic devices and sensitive probing in the life science and environment fields. In this study, N doped CDs-CdTe quantum dots (QDs) nanohybrids (CdTe/N-CDs) were synthesized by a plasma heating process using silk fibroin and CdTe QDs as precursors. The synthesis, doping, hybridizing and passivation of the CdTe/N-CDs were carried out in a single-step process. The as-synthesized CdTe/N-CDs dispersed in ethanol exhibited blue-violet photoluminescence with excitation-independent emission characteristics (strong emissions at 405 nm and 429 nm, and a weak emission at 456 nm). Additionally, the optimal excitation wavelength for the CdTe/N-CDs was found at 360–380 nm, which very closely matches the intrinsic wavelength of GaN-based LEDs. Furthermore, the obtained CdTe/N-CDs exhibited a very high quantum yield of ∼84%, showing great potential in developing chip-based high performance optoelectronics devices. The emission mechanism and emission enhancement by related factors including N-bonded configurations in the carbon base and the transfer of photo-excited electrons from the CdTe QDs to the N doped CDs were studied, as well.

The photoluminescence quantum yield of N-doped carbon dots was brought up to 84% by hybridizing with CdTe QDs.     

Rapidly detecting antibiotics with magnetic nanoparticle coated CdTe quantum dots

A reusable magnetic-quantum dot material (MNP–SiO₂–QD) with good magnetic properties and high fluorescence retention was successfully fabricated from linked magnetic nanoparticles and quantum dots. The resulting material can qualitatively and quantitatively detect four kinds of antibiotics and maintain high recovery rates.

A reusable magnetic-quantum dot material with good magnetic property and high fluorescence retention can qualitatively and quantitatively detect four kinds of antibiotics.

Antibiotics are essential low molecular weight chemicals that are generally used for therapeutic and prophylactic purposes in the livestock and poultry industries.¹ Also, veterinary drugs can also promote weight growth and improve breeding efficiency. However, indiscriminate usage has led to the accumulation of drugs in animal tissues and organs, which might lead to allergic reactions and bacteria resistance.² Furthermore, drug residues can also be present in milk, eggs and other animal products. These drug residues can be toxic to humans and threaten their health if present above certain levels.^3–5 Therefore, drug residues are an important issue in the field of food and animal feed safety.In order to protect human health and ensure food security, the European Union (EU) has established maximum drug residue limits in livestock.⁶ Various methods are used to detect these veterinary residues, including high-performance liquid chromatography (HPLC),⁷ mass spectrometry (MS)⁸ and capillary electrophoresis (CE).⁹ However, these methods require expensive equipment and time-consuming procedures. Solid-phase extraction (SPE) is generally used to detect medical residues,¹⁰ however, this method is not sensitive enough. Although enzyme-linked immunosorbent assays can rapidly analyze drug residues, they cannot be used to monitor multiple residues simultaneously.¹¹ Thus, the development of rapid multiplex detection methods for drug residues is necessary and urgent for regulating and monitoring food safety. Recently, nanomaterials have been used as candidates for residue detection. Quantum dots (QDs) have attracted attention due to their electrical and fluorescence properties.^12–14 For example, QDs have been applied to detect drug residues via electrochemiluminescence because of their electrical characteristics.^15–18 Li''s group¹⁵ developed an electrochemiluminescence immunosensor based on CdSe QDs that could be a good way to analyze salbutamol residues. Zhou''s group¹⁶ demonstrated an electrochemical protocol for detecting multiple antibiotics and their residues in milk samples, employing QDs as electrochemical tags that could reflect the identities and concentrations of veterinary residues. However, these electrochemiluminescence methods require complicated synthetic procedures and electrochemical equipment. It is necessary to establish more rapid, accurate and sensitive assay methodologies. To this end, the fluorescence properties of QD create the opportunity for drug residue detection.^19–25 Walia and Acharya²⁵ synthesized CdS QDs coated with glutathione for the selective detection of dicofol. Dicofol can interact with –COOH and –NH₂ in glutathione. The interaction between dicofol and glutathione can increase the fluorescence intensity. Furthermore, Li''s group²⁶ fabricated a core–shell complex with CdTe QDs and silicon spheres that was used as a fluorescence probe for detecting pesticides. Fluorescence detection has the advantages of high sensitivity, being simple and rapid, and needing no special testing equipment. However, this method is limited by its recycling properties. Therefore, an important development direction is achieving reusable quantum dots in the field of antibiotic detection. Magnetic nanoparticles have drawn interest due to their high magnetic responsiveness, easy separation and low toxicity, and they could be the best choice for achieving reusability. In the reported literature, magnetic materials have weak fluorescence intensities,²⁷ and studies involving the detection of drug residues using quantum dot fluorescence properties are rare.^28–30Therefore, considering the perfect fluorescence properties of quantum dots and the magnetic properties of Fe₃O₄, we have successfully fabricated a magnetic-quantum dot material (MNP–SiO₂–QD) with high fluorescence retention and reusability based on the properties of typical CdTe QDs and magnetic nanoparticles (MNPs). In this work, the fluorescence intensity of MNP–SiO₂–QD could reach 1300, which is significantly higher than other reported magnetic materials,²⁷ and the fluorescence retention rate could reach 60%. At the same time, MNP–SiO₂–QD can be reused because of its magnetic responsiveness. After five cycles of use, the fluorescence intensity of MNP–SiO₂–QD could still retain 50% of its initial level. In addition, the material can qualitatively and quantitatively detect antibiotics within 5 minutes, and detection had no dependence on the substrate used. Therefore, MNP–SiO₂–QD showed good prospects in the field of veterinary drug residue detection, providing a template for the development of new antibiotic detection methods, and ensuring the development of food safety.We constructed a kind of quantum dot material coated with magnetic nanoparticles, possessing excellent fluorescence properties and magnetic responsiveness, which could be used to detect antibiotics within 5 minutes. This material not only showed an improvement in the fluorescence intensity compared to other magnetically coated quantum dots, but it also achieved good recycling performance, showing potential for use in the field of rapid veterinary drug residue detection.     

Sensitive and accurate detection of ALP activity using a fluorescence on–off–on switch and mass barcode signal amplification

Alkaline phosphatase (ALP) is an important biomarker for many diseases. Therefore, the sensitive and accurate detection of ALP activity is essential for fundamental biochemical processes and clinic diagnosis. Herein, we design a fluorescent on–off–on switch for sensitive and visual detection of ALP activity. Meanwhile, mass barcode-modified quantum dots (QDs) amplified the LC-MS/MS detection signal in complex biological samples. Firstly, the QDs were modified with phosphorylated Gly-Gly-Phe-Phe-Tyr (OPO₃H₂) peptide (GGFFYp) and the mass barcode. The fluorescence of QDs-SS-Yp was quenched by fluorescence resonance energy transfer (FRET) between QDs-SS-Yp and dansyl chloride (DNS). ALP can hydrolyze the phosphorylated peptide to form peptide self-assemblies on the QDs-SS-Yp surfaces. The effective separation distance between the QDs-SS-Yp donor and DNS acceptor becomes larger, restricting FRET between the QDs-SS-Yp and DNS. At this point, the obvious QDs-SS-Yp fluorescence signal can be restored. However, the absence of ALP results in no peptide self-assembly on the QDs-SS-Yp surface and no obvious QDs-SS-Yp fluorescence signal was detected. Therefore, the ALP activity can be analyzed according to the degree of fluorescence restoration by the fluorescence on–off–on switch. Finally, the small tag molecules obtained by cleaving the disulfide bond of the QDs-SS-Yp as a mass barcode were used to amplify the LC-MS/MS detection signal. The proposed approach shows a good linear relationship (from 0.01 to 2.4 U L⁻¹) and has the significant advantage of a low detection limit of 0.001 U L⁻¹.

The sensitive and accurate detection of ALP activity using a fluorescence on–off–on switch and mass barcode signal amplification.     

A simple electrochemical immunosensor based on a chitosan/reduced graphene oxide nanocomposite for sensitive detection of biomarkers of malignant melanoma

The sensitive and specific detection of tumor biomarkers is crucial for early diagnosis and treatment of malignant melanoma. Immunoassay with a simple sensing interface and high sensitivity is highly desirable. In this work, a simple electrochemical immunosensor based on a chitosan/reduced graphene oxide (CS–rGO) nanocomposite was developed for sensitive determination of an S-100B protein, a tumor marker of malignant melanoma. CS–rGO nanocomposite were prepared by chemical reduction of graphene oxide in the presence of chitosan and modified on glassy carbon electrode (GCE) to provide a biofriendly, conductive, and easily chemically modified matrix for further immobilization of antibodies. Anti-S-100B antibodies were grafted onto the chitosan molecules to fabricate the immunorecognition interface by a simple glutaraldehyde cross-linking method. Electrochemical determination of S-100B was achieved by measuring the decreased current signal of solution phase electrochemical probes, which originated from the increased steric hindrance and insulation caused by the formation of antigen–antibody complexes at the electrode interface. Due to the good conductivity, high surface area, excellent biocompatibility, and good film-forming ability of CS–rGO, the constructed immunosensor exhibited good stability, high selectivity and sensitivity, a wide dynamic range from 10 fg mL⁻¹ to 1 ng mL⁻¹ and a low limit of detection of 1.9 pg mL⁻¹ (S/N = 3). Moreover, the sensor was also applicable for the sensitive detection of S-100B protein in real human serum samples.

Simple electrochemical immunosensor is easily fabricated based on chitosan/reduce graphene oxide nanocomposite for sensitive determination of a tumor marker of malignant melanoma.     

“Turn on” room-temperature phosphorescent biosensors for detection of hyaluronic acid based on manganese-doped ZnS quantum dots

Biosensors based on excellent optical properties of quantum dots (QDs) nanohybrids are efficient for biological detection. In this work, a room-temperature phosphorescent (RTP) PDAD–Mn–ZnS QDs biosensor was constructed with poly(diallyldimethylammonium chloride) (PDAD) as the modifier of MPA-capped Mn–ZnS QDs, and used to detect hyaluronic acid (HA). The newly-added HA induced severe electrostatic interaction with PDAD–Mn–ZnS QDs, leading to the aggregation between PDAD–Mn–ZnS QDs and HA and thereby enhancing RTP. The enhancement of RTP was proportional to the HA concentrations within certain ranges. On this basis, a high-performance HA sensor was built and this sensor had a detection limit of 0.03 μg mL⁻¹ and a detection range of 0.08–2.8 μg mL⁻¹. This proposed RTP sensor can avoid interferences from the background fluorescence or scattering light of the matrix that are encountered in spectrofluorometry. Thus, this biosensor is potentially suitable for detection of HA in real samples without complicated pretreatment.

Fabricating PDAD–Mn–ZnS QDs nanohybrids as a facile room-temperature phosphorescent biosensor for detection of hyaluronic acid.