Size-controlled sensitivity and selectivity for the fluorometric detection of Ag+ by homocysteine capped CdTe quantum dots

Two-photon excited luminescence of sulfur quantum dots for heavy metal ion detection

Spectrally-resolved third-order nonlinear optical properties of water-dispersed sulfur quantum dots (SQDs) were investigated in the wavelength range from 740 nm to 820 nm with the two-photon excited emission technique using a tunable femtosecond laser system. The maximum value of the two-photon absorption (TPA) cross-section (σ2) for ∼5.4 nm size SQDs was found to be 185 GM (Goeppert-Mayer unit), while the two-photon brightness (σ2 × η) was found to be 1.5 GM at 780 nm, the wavelength being in the first biological transmittance window. The TPA properties are presented here as appropriate cross-sections normalized per molecular weight which enables meaningful comparison of the nonlinear factors of the studied quantum dots with those of various nanomaterials. The optimized TPA properties of these hydrophilic colloidal SQDs may be potentially useful for detection of Fe3+ metal ions. The experimentally determined limit of Fe3+ detection for both one- and two-photon regime was 10 μmol L-1 (0.6 μg mL-1). Förster resonance energy transfer between SQDs as donors and Fe3+ metal ions as acceptors was confirmed as one of the possible detection mechanisms using a time-correlated single photon counting technique.

Highly sensitive and specific detection of silver ions using a dual-color fluorescence co-localization strategy

Ag⁺ ions are widely used in various fields of human life due to their unique properties and they threaten the environment and human health. The traditional methods for Ag⁺ detection commonly suffer from disadvantages including limited sensitivity, expensive equipment and complicated operating steps. Herein, we developed a highly specific dual-color fluorescence co-localization (DFC) strategy based on the C-Ag⁺-C structure for Ag⁺ detection. In this strategy, Ag⁺ ions can be captured to form C-Ag⁺-C base pairs, and these ions enable single-stranded DNAs to form double strands. The DFC strategy can exclude nonspecific interaction sites and greatly improve the sensitivity and specificity. By DFC of the QDs and Cy5 linked to the DNA strands, highly sensitive Ag⁺ detection was achieved in the concentration range from 0.14 pM to 200 nM, with a limit of detection (LOD) of 0.14 pM. Moreover, this method has been applied for the detection of Ag⁺ ions in real environmental samples with satisfactory recoveries. We believe that the DFC strategy is promising for Ag⁺ detection.

Colorimetric Detection of Metals Using CdS-NPs Synthesized by an Organic Extract of Aspergillus niger

The use of cadmium sulfide nanoparticles (CdS-NPs) synthesized by fungi presents highly stable chemical and optical characteristics; this makes them a promising alternative for development of colorimetric methods for metal detection. Moreover, application of CdS-NPs is challenging due to the biological material used to carry out synthesis and coating is highly diverse; therefore, it is necessary to evaluate if such components are present in the biological material. Thus, the objective of this work was to detect metallic ions in synthetic water samples using CdS-NPs synthesized by the extract of Aspergillus niger. The conditions to produce fungal extracts were determined through a factorial design 2³; additionally, biomolecules involved in metallic ions detection, synthesis, and coating of CdS-NPs were quantified; the studied biomolecules are NADH, sulfhydryl groups, proteins, and ferric reducing antioxidants (FRAP). CdS-NPs synthesized in this study were characterized by spectrophotometry, zeta potential, and high-resolution transmission electron microscopy (HRTEM). Finally, detection capacity of metallic ions in synthetic water samples was evaluated. It was proved that the methanolic extract of Aspergillus niger obtained under established conditions has the necessary components for both synthesis and coating of CdS-NPs, as well as detection of metallic ions because it was possible to synthesize CdS-NPs with a hexagonal crystalline structure with a length of 2.56 ± 0.50 nm which were able to detect Pb²⁺, Cr⁶⁺, and Fe³⁺ at pH 4 and Co²⁺ at pH 8.

Ratiometric fluorescent nanoprobes based on carbon dots and multicolor CdTe quantum dots for multiplexed determination of heavy metal ions

Owing to the high risk to human and environmental health, heavy metal pollution has become a global problem. Rapid, accurate and multiplexed determination of heavy metal ions is critical. In this work, we reported a promising approach to designing ratiometric fluorescent nanoprobes for multiplexed determination of Hg²⁺, Cu²⁺, and Ag⁺ ions. The nanoprobes (CDs-QDx) were designed by mixing the CDs and multicolor CdTe QDs without the involvement of recognition elements. The CDs were insensitive to heavy metal ions while CdTe QDs showed the size-dependent fluorescence response to different heavy metal ions, thereby establishing a ratiometric detection scheme by measuring the fluorescence intensity ratios of CDs-QDx systems. By evaluating the detection performance, the CDs-QDx (x = 570, 650, and 702) were successfully used for differentiation and quantification of Hg²⁺, Cu²⁺, and Ag⁺ ions. In addition, we also carried out the detection of heavy metal ions in actual samples with acceptable results. We believed that this work offers new insight into the design of ratiometric fluorescent nanoprobe for multiplexed determination of not only heavy metals but also some other analytes by combining the CDs with CdTe QDs with fine-tuned sizes.

An amplified fluorescent biosensor for Ag+ detection through the hybridization chain reactions

Ag is widely distributed in nature and it is used in almost all areas of human life. However, due to the widespread use of Ag materials, Ag⁺ pollution seriously threatens the human health and environment. The traditional detection methods for Ag⁺ suffer from disadvantages including high operational cost, complicated operating unit and instrument, and high requirements for professionals. Thus, in this study, a new type of Ag⁺ detection biosensor based on the hybridization signal amplification was designed to overcome these problems. Combining cytosine-Ag⁺-cytosine mismatch structure with the hybridization chain reaction, this biosensor converted the conventional detection signal into the nucleic acid amplification signal, which realized efficient, rapid, sensitive, and specific detection of Ag⁺. The limit-of-detection of this sensor reached 0.69 pM, which is much less than the maximum concentration (0.1 mg L^-1, 927 nM) suggested for drinking water by the World Health Organization, and the maximum concentration (0.05 mg L^-1, 464 nM) suggested by the United States Environmental Protection Agency. This method provides a promising new platform for detecting Ag⁺ concentrations at ultralow levels.

Solution-gated transistors of two-dimensional materials for chemical and biological sensors: status and challenges

Two-dimensional (2D) materials have been the focus of materials research for many years due to their unique fascinating properties and large specific surface area (SSA). They are very sensitive to the analytes (ions, glucose, DNA, protein, etc.), resulting in their wide-spread development in the field of sensing. New 2D materials, as the basis of applications, are constantly being fabricated and comprehensively studied. In a variety of sensing applications, the solution-gated transistor (SGT) is a promising biochemical sensing platform because it can work at low voltage in different electrolytes, which is ideal for monitoring body fluids in wearable electronics, e-skin, or implantable devices. However, there are still some key challenges, such as device stability and reproducibility, that must be faced in order to pave the way for the development of cost-effective, flexible, and transparent SGTs with 2D materials. In this review, the device preparation, device physics, and the latest application prospects of 2D materials-based SGTs are systematically presented. Besides, a bold perspective is also provided for the future development of these devices.

Functional Carbon Quantum Dots for Highly Sensitive Graphene Transistors for Cu2+ Ion Detection

Cu²⁺ ions play essential roles in various biological events that occur in the human body. It is important to establish an efficient and reliable detection of Cu²⁺ ions for people's health. The solution-gated graphene transistors (SGGTs) have been extensively investigated as a promising platform for chemical and biological sensing applications. Herein, highly sensitive and highly selective sensor for Cu²⁺ ion detection is successfully constructed based on SGGTs with gate electrodes modified by functional carbon quantum dots (CQDs). The sensing mechanism of the sensor is that the coordination of CQDs and Cu²⁺ ions induces the capacitance change of the electrical double layer (EDL) near the gate electrode and then results in the change of channel current. Compared to other metal ions, Cu²⁺ ions have an excellent binding nature with CQDs that make it an ultrahigh selective sensor. The CQD-modified sensor achieves excellent Cu²⁺ ion detection with a minimal level of concentration (1 × 10^-14 M), which is several orders of magnitude lower than the values obtained from other conventional detection methods. Interestingly, the device also displays a quick response time on the order of seconds. Due to the functionalized nature of CQDs, SGGTs with CQD-modified gate show good prospects to achieve multifunctional sensing platform in biochemical detections.

Development of a portable device for Ag+ sensing using CdTe QDs as fluorescence probe via an electron transfer process

Ag⁺ as one of the most commonly seen toxic heavy metal ions is involved in numerous vital biological processes, which would cause fatal damages and environmental contamination when Ag⁺ is excessive. In the present work, CdTe quantum dots (QDs) with green, orange, and red emission capped by mercaptoacetic acid (TGA) were synthesized at one time by controlling the synthesis time and utilized for Ag⁺ detection. Both fluorescence spectral red-shift and intensity decrease could be used for Ag⁺ discrimination. Fluorescence lifetime, Zeta potential, and XRD, etc. were carried out to analyze the detection mechanism. Results displayed that surface passivation and electron transfer due to binding effects of Ag⁺ to Te atom on traps of QDs could be relied on to explain the sensing mechanism. Additionally, in accordance with PCA analysis, Ag⁺ could be also be successfully differentiated from Hg²⁺ and the other metal ions. Importantly, a home-made portable device based on a 32 bit embed Micro Control Unit (MCU) system was first proposed for Ag⁺ detection. The power supply system adopt the mini-sized lithium cell instead of the power supply system, which ensure its practical applicability. The relative position of light source and detector is set at 90° to minimize the interference. According to the detection results, the linear detection range using the device was from 5 nM to 200 nM (with a larger coefficient of determination, R²), and the detection limit was calculated to be about 5 nM, which indicated that this proposed method and device could fulfil the practical application requirements.

Analyte-triggered autocatalytic amplification combined with gold nanoparticle probes for colorimetric detection of heavy-metal ions

This work reports a new colorimetric nanosensor for the detection of heavy-metal ions that initially integrates analyte-triggered autocatalytic amplification with o-phenylenediamine-mediated aggregation of label-free gold nanoparticles. Its utility is well demonstrated with the simple, rapid, sensitive, and specific detection of Hg²⁺, Cu²⁺, and Ag⁺ targets with detection limits less than 3 nM.

One-pot synthesis of highly luminescent N-acetyl-l-cysteine-capped CdTe quantum dots and their size effect on the detection of glutathione

N-Acetyl-l-cysteine-capped CdTe QDs with a large size (3.68 nm) are more suitable for detecting glutathione than the small ones (1.99 nm).

CdTe quantum dot-based fluorescent probes for selective detection of Hg (II): The effect of particle size

Mercury ions-induced fluorescence quenching properties of CdTe quantum dots (QDs) have been studied using the fluorescence spectroscopic techniques. By using the hydrothermal method, the CdTe QDs with different particles sizes from 1.98 to 3.68nm have been prepared, and the corresponding fluorescence emission wavelength is changed from 518 to 620nm. The fluorescence of QDs is enhanced after linking Bovine serum albumin (BSA) onto the surface of the QDs. Experimental results show that the fluorescence intensity of BSA-coated CdTe QDs could be effectively quenched when Hg²⁺ react with BSA-coated CdTe QDs. Interestingly, both the sensing sensitivity and selectivity of this fluorescence probe could be improved when the particle size of the QDs decreases. Thus the BSA-coated CdTe QDs with green fluorescence emission have better advantages than the BSA-coated CdTe QDs with red fluorescence for Hg²⁺ detection. Interference experiment results indicate that the influence from other metal ions could be neglected in the detection, and the Hg²⁺ could be specifically detected. By using this BSA-coated CdTe QDs-based fluorescence probe, the Hg²⁺ could be detected with an ultra-low detection limit of nanomole level, and the linear range spans a scope from 0.001 to 1μmol/L.

Recent progress in quantum dot based sensors

Recent progress in quantum dot (QD) based chemo- and biosensors for various applications is summarized.