Polychromatic fluorescent MoS2 quantum dots: fabrication and off-on sensing for fluorine ions in water

Recently, MoS₂ quantum dots (QDs) have receive widespread attention as a promising luminescent material. However, so far, little effort has been made on the multicolor emission of MoS₂ QDs. Herein, an in situ iodine doping strategy is presented and used to synthesize tunable-photoluminescent (PL) MoS₂ QDs. By fine iodine doping, the PL of the MoS₂ QDs (I-MoS₂ QDs) can be tuned in the range from 423 nm to 529 nm, which exceeds the as-reported emission wavelength range. Studies using controlled experiments and density functional theory (DFT) reveal that the change in electronic state of MoS₂ QDs is responsible for the changing PL due to iodine doping. As-synthesized I-MoS₂ QDs combined with Fe³⁺ is developed as a "turn-off-on" fluorescence sensor for F^- ions in water. The fluorescence probe has a fine linear response to F^- ions in the concentration range of 2.5-80 μM, and the limit of detection is 1.4 μM (S/N = 3).

Conventional and advanced detection approaches of fluoride in water: a review

Fluorine is a naturally occurring element found in soil, water, food materials, and natural minerals such as fluorapatite, sellaite, and cryolite and exists as fluoride compounds with other elements because of high reactivity. The exposure of fluoride to the environment and human beings are industrial factors, food, water, and geogenic factors that impact the health of millions of human beings worldwide. Overexposure to fluoride exceeding the permissible limit (1.5 mg/l as per WHO) causes several diseases in human beings, such as teeth mottling, thyroid inflammation, dental fluorosis, skeletal fluorosis, lesions in the kidney, and other organs. To overcome the deleterious impact of fluoride, its detection at an early stage is very much required. Therefore, feeling the importance of the same, immense efforts have been made to the selective and sensitive determination of fluoride in water by numerous researchers. This review paper summarizes the various conventional methods such as spectroscopic, ion chromatography, ICP-OES, and gas chromatography-mass spectrometry, their advantages, and drawbacks leading to the development of advanced ready-to-use detection strategies such as stamartphones for on-the-spot fluoride detection. This review paper also discusses future directions, which will assist scientists in achieving a new benchmark in developing a reliable, cost-effective, and user-friendly fluoride detector.

Design and synthesis of stable indigo polymer semiconductors for organic field-effect transistors with high fluoride sensitivity and selectivity

We report the design and synthesis of two novel indigo donor-acceptor (D-A) polymers, PIDG-T-C20 and PIDG-BT-C20, comprising an indigo moiety that has intramolecular hydrogen-bonds as the acceptor building block and thiophene (T) and bithiophene (BT) as the donor building block, respectively. PIDG-T-C20 and PIDG-BT-C20 exhibited characteristic p-type semiconductor performance, achieving hole mobilities of up to 0.016 and 0.028 cm2 V-1 s-1, respectively, which are highest values reported for indigo-based polymers. The better performing PIDG-BT-C20 was used for the fabrication of water-gated organic field-effect transistors (WGOFETs), which showed excellent stability at ambient conditions. The PIDG-BT-C20-based WGOFETs exhibited rapid response when fluoride ions were introduced to the water gate dielectric, achieving a limit of detection (LOD) of 0.40 mM. On the other hand, the devices showed much lower sensitivities towards other halide ions with the order of relative response: F- ≫ Cl- > Br- > I-. The high sensitivity and selectivity of PIDG-BT-C20 to fluoride over other halides is considered to be realized through the strong interaction of the hydrogen atoms of the N-H groups in the indigo unit with fluoride ions, which alters the intramolecular hydrogen-bonding arrangement, the electronic structures, and thus the charge transport properties of the polymer.

Sensing with photoluminescent semiconductor quantum dots

Fluorescent sensors benefit from high signal-to-noise and multiple measurement modalities, enabling a multitude of applications and flexibility of design. Semiconductor nanocrystal quantum dots (QDs) are excellent fluorophores for sensors because of their extraordinary optical properties. They have high thermal and photochemical stability compared to organic dyes or fluorescent proteins and are extremely bright due to their large molar cross-sections. In contrast to organic dyes, QD emission profiles are symmetric, with relatively narrow bandwidths. In addition, the size tunability of their emission color, which is a result of quantum confinement, make QDs exceptional emitters with high color purity from the ultra-violet to near infrared wavelength range. The role of QDs in sensors ranges from simple fluorescent tags, as used in immunoassays, to intrinsic sensors that utilize the inherent photophysical response of QDs to fluctuations in temperature, electric field, or ion concentration. In more complex configurations, QDs and biomolecular recognition moieties like antibodies are combined with a third component to modulate the optical signal via energy transfer. QDs can act as donors, acceptors, or both in energy transfer-based sensors using Förster resonance energy transfer (FRET), nanometal surface energy transfer (NSET), or charge or electron transfer. The changes in both spectral response and photoluminescent lifetimes have been successfully harnessed to produce sensitive sensors and multiplexed devices. While technical challenges related to biofunctionalization and the high cost of laboratory-grade fluorimeters have thus far prevented broad implementation of QD-based sensing in clinical or commercial settings, improvements in bioconjugation methods and detection schemes, including using simple consumer devices like cell phone cameras, are lowering the barrier to broad use of more sensitive QD-based devices.

Anion induced supramolecular polymerization: a novel approach for the ultrasensitive detection and separation of F−

A novel approach for the ultrasensitive detection and separation of F⁻ has been developed. F⁻ could induce a tripodal naphthalimide sensor (TNA) to carry out reversible supramolecular polymerization and lead to strong AIEE.

Nanomaterials for the optical detection of fluoride

Overexposure to fluoride ions (F^-) causes serious diseases in human beings. Extensive efforts have been made to develop sensitive and selective approaches for F^- detection and a variety of F^- sensors have been constructed recently. The burgeoning nanotechnology has provided novel materials for F^- analysis due to the extraordinary properties of nanomaterials. In this review, we present the recent advances in different nanomaterials-based approaches for the optical F^- detection via colorimetric, fluorescent and chemiluminescent responses. The materials include gold nanomaterials, CeO₂ nanoparticles, semiconductor quantum dots, carbon quantum dots, metal-organic frameworks, upconversion nanoparticles, micellar nanoparticles, polymer dots, SiO₂ nanoparticles and graphene oxide. The recent trends and challenges in the optical detection of F^- with various nanomaterials are also discussed.

Rare earth ions enhanced near infrared fluorescence of Ag2S quantum dots for the detection of fluoride ions in living cells

In this work, a novel phenomenon was discovered that the fluorescence intensity of silver sulfide quantum dots (Ag₂S QDs) could be enhanced in the presence of rare earth ions through aggregation-induced emission (AIE). Based on the strong coordination between rare earth ions and F^-, a facile and label-free strategy was developed for the detection of F^- in living cells. Ag₂S QDs were synthesized using 3-mercaptopropionic acid as sulfur source and stabilizer in aqueous solution. The near infrared (NIR) emitting QDs exhibited excellent photostalilty, high quantum yield and low toxic. Interestingly, the fluorescence intensity of QDs was obviously enhanced upon the addition of various rare earth ions, especially in the presence of Gd³⁺. The AIE mechanism was proved via the TEM, zeta potential and dynamic light scattering analysis. Moreover, the coordination between rare earth ions and F^- could lead to the quenching of fluorescence QDs due to the weakening the AIE. Based on these findings, we developed a highly sensitive and selective method for detection of F^-. The label-free NIR fluorescence probe was successfully used for F^- bioimaging in live cells.

Ultrasensitive optical detection of anions by quantum dots

Quantum dots (QDs) have received great interest for diverse applications over the past few decades due to their unique photophysical properties like their tunable band gap, facile solution processability and versatile surface functionalization with different ligands. Quantum dot based optical analysis techniques with high sensitivity and selectivity have been developed to detect anions in aqueous solution for environmental monitoring, medicinal diagnostics, and the analysis of biological samples and industrial processes. Here we review the latest research progress of semiconductor QDs for sensing of anions in aqueous solution or in vivo, and discuss the photophysical mechanisms and outlook for the potential development in QD based optical sensing for anions.

Fluorometric selective detection of fluoride ions in aqueous media using Ag doped CdS/ZnS core/shell nanoparticles

In this study, a fluorometric method based on Ag–CdS/Ag–ZnS core/shell nanoparticles is developed for fluoride ion detection.

A facile method to effectively combine plasmon enhanced fluorescence (PEF) and fluoride-Lewis acid based reactions to detect low concentrations of fluoride in solution

Synergizing the Purcell-effect of silver nanoparticles and fluoride-Lewis acid based reactions to enhance the sensitivity and detection limit of a fluoride ion chemodosimeter.

Group 6 metal pentacarbonyl complexes of air-stable primary, secondary, and tertiary ferrocenylethylphosphines

The synthesis and characterization of a series of Group 6 metal pentacarbonyl complexes of air stable primary, secondary, and tertiary phosphines containing ferrocenylethyl substituents are reported [M(CO)5L: M = Cr, Mo, W; L = PH2(CH2CH2Fc), PH(CH2CH2Fc)2, P(CH2CH2Fc)3]. The structure and composition of the complexes were confirmed by multinuclear NMR spectroscopy, IR and UV-Vis absorption spectroscopy, mass spectrometry, X-ray crystallography, and elemental analysis. The solid-state structural data reported revealed trends in M-C and M-P bond lengths that mirrored those of the atomic radii of the Group 6 metals involved. UV-Vis absorption spectroscopy and cyclic voltammetry highlighted characteristics consistent with electronically isolated ferrocene units including wavelengths of maximum absorption between 435 and 441 nm and reversible one-electron (per ferrocene unit) oxidation waves between 10 and -5 mV relative to the ferrocene/ferrocenium redox couple. IR spectroscopy confirmed that the σ donating ability of the phosphines increased as ferrocenylethyl substituents were introduced and that the tertiary phosphine ligand described is a stronger σ donor than PPh3 and a weaker σ donor than PEt3, respectively.

Design of Fe₃O₄@SiO₂@Carbon Quantum Dot Based Nanostructure for Fluorescence Sensing, Magnetic Separation, and Live Cell Imaging of Fluoride Ion

A robust reusable fluoride sensor comprised of a receptor in charge of the chemical recognition and a fluorophore responsible for signal recognition has been designed. Highly fluorescent carbon quantum dot (CD) and magnetically separable nickel ethylenediaminetetraacetic acid (EDTA) complex bound-silica coated magnetite nanoparticle (Fe3O4@SiO2-EDTA-Ni) have been used as fluorophore and fluoride ion receptor, respectively. The assay is based on the exchange reaction between the CD and F(-), which persuades the binding of fluoride to magnetic receptor. This method is highly sensitive, fast, and selective for fluoride ion in aqueous solution. The linear response range of fluoride (R(2) = 0.992) was found to be 1-20 μM with a minimum detection limit of 0.06 μM. Excellent magnetic property and superparamagnetic nature of the receptor are advantageous for the removal and well quantification of fluoride ion. The practical utility of the method is well tested with tap water. Because of high sensitivity, reusability, effectivity, and biocompatibility, it exhibits great promise as a fluorescent probe for intracellular detection of fluoride.

Surface functionalized fluorescent CdS QDs: selective fluorescence switching and quenching by Cu(2+) and Hg(2+) at wide pH range

Fluorescent CdS QDs surface functionalized with organic functional unit, N-(2-hydroxybenzyl)-cysteine (L), has been synthesized using simple wet chemical approach in a single step. The as-prepared L-CdS QDs showed good fluorescence at 542 nm. Fluorescence sensor studies of L-CdS QDs showed selective fluorescence switching (542-600 nm) for Cu(2+) ions and quenching for Hg(2+) ions in aqueous solution. High selectivity of L-CdS QDs for Cu(2+) ions have been confirmed by interference studies. Interestingly, L-CdS QDs showed enhancement in the fluorescence intensity (4-fold) upon reducing pH from 2.0 to 10.0 without changing λmax. Importantly, selective fluorescent switching and quenching for Cu(2+) and Hg(2+) was observed from 2.0 to 10.0 pH range. The practical application of L-CdS QDs fluorescence sensing for Cu(2+) and Hg(2+) have also been demonstrated in real samples such as river, pond, tap and ground water.

A simple and effective 1,2,3-triazole based “turn-on” fluorescence sensor for the detection of anions

1,2,3-Triazole based chemosensor is synthesized using “Click chemistry” approach. Addition of fluoride ion “turn-on” the fluorescence response of probe.

Discriminative detection of bivalent Cu by dual-emission ZnSe quantum dot fluorescence sensing via ratiometric fluorescence measurements

In this work, we showed that 1-thioglycerol (TG)-capped ZnSe quantum dots (QDs) with dual-emission could perform ideal QD fluorescence sensing for ratiometric fluorescence measurements. By comparing the fluorescence ratios at two emission peaks before and after the addition of cations, the discriminative detection of Cu(II) was realized, even in the case of co-existing with large amounts of other sensitive cations, such as Ag(I). The discriminative detection of Cu(II) is accurate with co-existing Ag(I) below 10 μmol L(-1). By a joint investigation of the ionic diffuse dynamics and carrier recombination dynamics, we found that the adsorbed layer of QDs plays a key role in the discriminative detection of Cu(II) from Ag(I) or other sensitive cations. The moderate adsorption capacity with a QD adsorbed layer makes Cu(II) capable of travelling across the QD double-layer structure, following a surface doping process via chemical reactions between Cu(II) and the QD surface atoms. As a result of Cu(II) doping, there were three major carrier recombination channels: the non-radiation recombination between the QD conduction band to the Cu(II) energy level, together with the non-radiation recombination and radiation recombination between the trap state energy levels and the Cu(II) energy level. As for Ag(I) and other sensitive cations, they have a strong adsorption capacity with the QD adsorbed layer, making them mainly present on the adsorbed layer. Due to the blocking of the ligand layer, we only observed weak coupling of the ZnSe conduction band with the Ag(I) energy level via a non-radiation recombination channel.

Efficient energy transfer from inserted CdTe quantum dots to YVO₄:Eu³⁺ inverse opals: a novel strategy to improve and expand visible excitation of rare earth ions

Rare earth (RE)-based phosphors demonstrate sharp emission lines, long lifetimes and high luminescence quantum yields; thus, they have been employed in various photoelectric devices, such as light-emitting diodes (LEDs) and solar spectral converters. However, their applications are largely confined by their narrow excitation bands and small absorption cross sections of 4f-4f transitions. In this paper, we demonstrate a novel strategy to improve and expand the visible excitation bands of Eu(3+) ions through the interface energy transfer (ET) from CdTe quantum dots (QDs) to YVO₄:Eu(3+) inverse opal photonic crystals (IOPCs). The significant effects observed in the CdTe QDs/YVO₄:Eu(3+) IOPCs composites were that the excitation of Eu(3+) ions was continuously extended from 450 to 590 nm and that the emission intensity of the (5)D₀-(7)FJ transitions was enhanced ∼20-fold, corresponding to the intrinsic (7)F₁-(5)D₁ excitation at 538 nm. Furthermore, in the IOPC network, the ET efficiency from the QDs to YVO₄:Eu(3+) was greatly improved because of the suppression of energy migration among the CdTe QDs, which gave an optimum ET efficiency as high as 47%. Besides, the modulation of photonic stop bands (PSBs) on the radiative transition rates of the QDs and Eu(3+) ions was studied, which showed that the decay lifetime constants for Eu(3+) ions were independent of PSBs, while those of QDs demonstrated a suppression in the PSBs. Their physical nature was explained theoretically.

Light harvesting of CdSe/CdS quantum dots coated with β-cyclodextrin based host–guest species through resonant energy transfer from the guests

Nano-hybrids based on red emitting QDs covered by β-cyclodextrin hosting a green emitting nitrobenzoxadiazole derivative show emission harvested by the host–guest organic system.

A novel ascorbic acid sensor based on the Fe3+/Fe2+ modulated photoluminescence of CdTe quantum dots@SiO2 nanobeads

In this paper, CdTe quantum dot (QD)@silica nanobeads were used as modulated photoluminescence (PL) sensors for the sensing of ascorbic acid in aqueous solution for the first time. The sensor was developed based on the different quenching effects of Fe(2+) and Fe(3+) on the PL intensity of the CdTe QD@ silica nanobeads. Firstly, the PL intensity of the CdTe QDs was quenched in the presence of Fe(3+). Although both Fe(2+) and Fe(3+) could quench the PL intensity of the CdTe QDs, the quenching efficiency were quite different for Fe(2+) and Fe(3+). The PL intensity of the CdTe QD@silica nanobeads can be quenched by about 15% after the addition of Fe(3+) (60 μmol L(-1)), while the PL intensity of the CdTe QD@silica nanobeads can be quenched about 49% after the addition of Fe(2+) (60 μmol L(-1)). Therefore, the PL intensity of the CdTe QD@silica nanobeads decreased significantly when Fe(3+) was reduced to Fe(2+) by ascorbic acid. To confirm the strategy of PL modulation in this sensing system, trace H2O2 was introduced to oxidize Fe(2+) to Fe(3+). As a result, the PL intensity of the CdTe QD@silica nanobeads was partly recovered. The proposed sensor could be used for ascorbic acid sensing in the concentration range of 3.33-400 μmol L(-1), with a detection limit (3σ) of 1.25 μmol L(-1) The feasibility of the proposed sensor for ascorbic acid determination in tablet samples was also studied, and satisfactory results were obtained.

Zn(x)Cd(1-x)Se nanomultipods with tunable band gaps: synthesis and first-principles calculations

In this paper, we demonstrate that ZnxCd1-xSe nanomultipods can be synthesized via a facile and nontoxic solution-based method. Interesting aspects of composition, morphology and optical properties were deeply explored. The value of Zn/(Zn+Cd) could be altered across the entire range from 0.08 to 0.86 by varying the ratio of cation precursor contents. The band gap energy could be linearly tuned from 1.88 to 2.48 eV with respect to the value of Zn/(Zn+Cd). The experiment also showed that oleylamine played a dominant role in the formation of multipod structure. A possible growth mechanism was further suggested. First-principles calculations of band gap energy and density of states in the Vienna ab initio simulation package code were performed to verify the experimental variation tendency of the band gap. Computational results indicated that dissimilarities of electronic band structures and orbital constitutions determined the tunable band gap of the as-synthesized nanomultipod, which might be promising for versatile applications in relevant areas of solar cells, biomedicine, sensors, catalysts and so on.

3-Aminophenyl boronic acid-functionalized CuInS2 quantum dots as a near-infrared fluorescence probe for the determination of dopamine

Water-soluble CuInS2 ternary quantum dots (QDs) capped by mercaptopropionic acid were directly synthesized in aqueous solution. Consequently, the CuInS2 QDs were covalently linked to 3-aminophenyl boronic acid molecules to form the 3-aminophenyl boronic acid-functionalized CuInS2 QDs (F-CuInS2 QDs). The F-CuInS2 QDs had a fairly symmetric fluorescence emission centered at 736nm that was in the near-infrared region (NIR). The F-CuInS2 QDs containing boronic acid functional groups were reactive toward vicinal diols to form five- or six-member cyclic esters in an alkaline aqueous solution. The reaction would cause the fluorescence quenching, which could be used as a fluorescence probe for the determination of dopamine (DA). This assay could also probe other vicinal diols such as catechol, pyrogallol, and gallate, based on the fluorescence quenching of the F-CuInS2 QDs, and this assay was nearly unaffected by other phenol compounds such as phenol, resorcinol, and hydroquinone without the vicinal diol structures. The developed F-CuInS2 QDs were applied to the detection of DA in human serum samples with satisfactory results. Therefore, this experment provided a simple and sensitve NIR fluorescence probe for the detection of DA, catechol, pyrogallol, and gallate.

Luminescent 'On-Off' CdSe/ZnS quantum dot chemodosimeter for hydroxide based on photoinduced electron transfer from a carboxylate moiety

A CdSe-ZnS quantum dot (QD) has been surface functionalised by a place exchange reaction with p-mercaptomethyl benzoate synthesized by a three-step procedure. The resulting lumophore-spacer-receptor QD-conjugate was characterized by IR, UV-visible and fluorescence spectroscopy. The emission profile of the QD reveals a narrow emission peak centred at 542 nm. Addition of hydroxide to the solution containing the QD-conjugate results in quenching of the original fluorescence, which is attributed to a photoinduced electron transfer reaction from the electron-rich benzoate moiety to the QD valence band. This is the first reported example of fluorescent quenching of a CdSe-ZnS QD luminescence by an aryl carboxylate moiety.

Detection of ascorbic acid and folic acid based on water-soluble CuInS2 quantum dots

In this paper, water-soluble CuInS(2) ternary quantum dots (QDs) modified by mercaptopropionic acid (MPA) were directly synthesized by hydrothermal method. Ascorbic acid (AA) can induce the fluorescence enhancement of MPA-capped CuInS(2) QDs and can be used for the detection of AA. Under the optimized conditions, the relationship between the fluorescence intensity of the CuInS(2) QDs and AA concentration was linear in the range of 0.25-200 μmol L(-1). Most relevant molecules and physiological ions had no effect on the detection of AA. The fluorescence intensity of CuInS(2) QDs enhanced by a certain amount of AA could be reduced in the presence of folic acid (FA) and thus can be used for the detection of FA with the linear range of 0.2-100 μmol L(-1). Compared with previous reports, the established approach utilized a simple, sensitive, and selective strategy to develop the QDs probe based on fluorescence enhancing and quenching phenomena without complicated immobilization.

Self-assembly of folate onto polyethyleneimine-coated CdS/ZnS quantum dots for targeted turn-on fluorescence imaging of folate receptor overexpressed cancer cells

Folate receptor (FR) can be overexpressed by a number of epithelial-derived tumors, but minimally expressed in normal tissues. As folic acid (FA) is a high-affinity ligand to FR, and not produced endogenously, development of FA-conjugated probes for targeted imaging FR overexpressed cancer cells is of significance for assessing cancer therapeutics and for better understanding the expression profile of FR in cancer. Here we report a novel turn-on fluorescence probe for imaging FR overexpressed cancer cells. The probe was easily fabricated via electrostatic self-assembly of FA and polyethyleneimine-coated CdS/ZnS quantum dots (PEI-CdS/ZnS QDs). The primary fluorescence of PEI-CdS/ZnS QDs turned off first upon the electrostatic adsorption of FA onto PEI-CdS/ZnS QDs based on electron transfer to produce negligible fluorescence background. The presence of FR expressed on the surface of cancer cells then made FA desorb from PEI-CdS/ZnS QDs due to specific and high affinity of FA to FR. As a result, the primary fluorescence of PEI-CdS/ZnS QDs adhering to the cells turned on due to the inhibition of electron transfer. The most important merits of the developed probe are its simplicity and the effective avoidance of the false positive results due to the simple electrostatic self-assembly of FA onto the surface of PEI-CdS/ZnS QDs and the involved fluorescence "off-on" mechanism. The probe was demonstrated to be sensitive and selective for targeted imaging of FR overexpressed cancer cells in turn-on mode.

Wide dynamic range sensing with single quantum dot biosensors

Single-particle analysis of biosensors that use charge transfer as the means for analyte-dependent signaling with semiconductor nanoparticles, or quantum dots, was examined. Single-particle analysis of biosensors that use energy transfer show analyte-dependent switching of nanoparticle emission from off to on. The charge-transfer-based biosensors reported here show constant emission, where the analyte (maltose) increases the emission intensity. By monitoring the same nanoparticles under various conditions, a single charge-transfer-based biosensor construct (one maltose binding protein, one protein attachment position for the reductant, one type of nanoparticle) showed a dynamic range for analyte (maltose) detection spanning from 100 pM to 10 μM while the emission intensities increase from 25 to 175% at the single-particle level. Since these biosensors were immobilized, the correlation between the detected maltose concentration and the maltose-dependent emission intensity increase could be examined. Minimal correlation between maltose detection limits and emission increases was observed, suggesting a variety of reductant-nanoparticle surface interactions that control maltose-dependent emission intensity responses. Despite the heterogeneous responses, monitoring biosensor emission intensity over 5 min provided a quantifiable method to monitor maltose concentration. Immobilizing and tracking these biosensors with heterogeneous responses, however, expanded the analyte-dependent emission intensity and the analyte dynamic range obtained from a single construct. Given the wide dynamic range and constant emission of charge-transfer-based biosensors, applying these single molecule techniques could provide ultrasensitive, real-time detection of small molecules in living cells.

Quantum dot-Eu3+ conjugate as a luminescence turn-on sensor for ultrasensitive detection of nucleoside triphosphates

We report a conjugate of thioglycolic acid (TGA) capped CdTe quantum dot and Eu(3+) ion (TGA-CdTe QD-Eu(3+)) that can be used as an ultrasensitive luminescence turn-on sensor for nucleoside triphosphates (NTPs). The TGA-CdTe QD-Eu(3+) conjugate is a weakly luminescent species as a result of the strong quenching effect of Eu(3+) ion on the luminescence of TGA-CdTe QDs. The conjugate's luminescence can be readily restored by its reaction with adenosine triphosphate (ATP) and other NTPs, and thus gives an ultrasensitive detection of NTPs, with a detection limit of 2 nM. The sensing mechanism has also been explored, and the effective quenching of TGA-CdTe QDs emission by Eu(3+) ions has been attributed to photoinduced electron transfer (PET). ATP, as the representative of NTPs, can remove Eu(3+) from the surface of TGA-CdTe QDs, leading to restoration of the TGA-CdTe QDs luminescence.

Recent advances and applications in QDs-based sensors

As a unique nanomaterial, quantum dots (QDs) are not only applied in fluorescent labeling and biological imaging, but are also utilized in novel sensing systems. Because QDs have attractive optoelectronic characteristics, QD-based sensors present high sensitivity in detecting specific analytes in the chemical and biochemical fields. In this review, we describe the basic principles and different conjugation strategies in QD-based sensors. An overview of recent advances and various models of QD-sensing systems is also provided. Furthermore, perspectives for sensors based on QDs are discussed.