Highly luminescent hybrid SiO2‐coated CdTe quantum dots: synthesis and properties

Novel hybrid SiO₂‐coated CdTe quantum dots (QDs) were created using CdTe QDs coated with a hybrid SiO₂ shell containing Cd²⁺ ions and a sulfur source via a sol–gel process in aqueous solution. Aqueous CdTe QDs with tunable emitting color created through a reaction between cadmium chloride and sodium hydrogen telluride was used as cores for the preparation of hybrid SiO₂‐coated CdTe QDs. In our experiments we found that the surface state of the cores and preparation conditions that affect the formation of the hybrid SiO₂ shell also greatly affect photoluminescence of the hybrid SiO₂‐coated CdTe QDs. The generation of CdS‐like clusters in the vicinity of the CdTe QDs, caused the quantum size effect of the QDs to be greatly reduced, which changes photoluminescence properties of the hybrid QDs fundamentally. Namely, the novel hybrid SiO₂ shell played an important role in generating a series of specific optical properties. In addition, the novel hybrid SiO₂ shell can be created if no CdTe QD is added. In order to gain an insight into the inter structure of the hybrid shell, we characterized the hybrid SiO₂‐coated CdTe QDs using X‐ray diffraction analysis and discuss the formation mechanism of such a hybrid structure. This work is significant because the novel hybrid SiO₂‐coated CdTe QDs with its excellent properties can be used in many applications, such as biolabeling and optoelectronic devices. Copyright © 2013 John Wiley & Sons, Ltd.     

Temperature Driven Plasmon-Exciton Coupling in Thermoresponsive Dextran-Graft-PNIPAM/Au Nanoparticle/CdTe Quantum Dots Hybrid Nanosystem

The temperature-driven plasmon-exciton coupling in thermoresponsive dextran-graft-PNIPAM/Au nanoparticle/CdTe quantum dot (D-g-PNIPAM/Au NPs/CdTe QDs) hybrid nanosystem was studied. A significant (0.84 eV) splitting of the absorption peak was observed in the absorption spectrum of the nanosystem, which reflects the fact of formation of plexcitons, occurring due to strong plasmon-exciton coupling. An increasing with time plasmonic enhancement of the photoluminescence of CdTe QDs was revealed, as a result of the penetration of quantum dots into the volume of the D-g-PNIPAM/Au NP hybrid nanosystem and bonding to it. The heating–cooling cycle of the aqueous solution of the studied nanosystem leads to a reversible quenching-recovery alteration of the QD photoluminescence. The quenching was rationalized as a result of an increased probability of nonradiative resonance energy transfer (RET) from CdTe QDs to Au NPs, which occurs due to shortening of the NP-QD distance, caused by shrinking of the macromolecule due to cooling-induced lower critical solution temperature phase transition. Increasing the NP-QD distance in the heating stage recovers the QD PL intensity. The observed effect opens up opportunities for the controlled reversible temperature-driven tuning of the photoluminescence intensity of D-g-PNIPAM/Au NP/CdTe QD nanosystem, which is highly important for its potential use in photonics and biomedical applications.

     

Synthesis of highly luminescent and biocompatible CdTe/CdS/ZnS quantum dots using microwave irradiation: a comparative study of different ligands

We compared the effects of several ligands frequently used in aqueous synthesis, including L‐cysteine, L‐cysteine hydrochloride, N‐acetyl‐L‐cysteine (NAC), glutathione and 3‐mercaptopropionic acid, for microwave synthesis of CdTe quantum dots (QDs) in a sealed vessel with varied temperatures and times, and then developed a rapid microwave‐assisted protocol for preparing highly luminescent, photostable and biocompatible CdTe/CdS/ZnS core–multishell QDs. The effects of molecular structures of these ligands on QD synthesis under high temperatures were explored. Among these ligands, NAC was found to be the optimal ligand in terms of the optical properties of resultant QDs and reaction conditions. The emission wavelength of NAC‐capped CdTe QDs could reach 700 nm in 5 min by controlling the reaction temperature, and the resultant CdTe/CdS/ZnS core–multishell QDs could achieve the highest quantum yields up to 74% with robust photostability. In addition, the effects of temperature, growth time and shell–precursor ratio on shell growth were examined. Finally, cell culturing indicated the low cytotoxicity of CdTe/CdS/ZnS core–multishell QDs as compared to CdTe and CdTe/CdS QDs, suggesting their high potential for applications in biomedical imaging and diagnostics. Copyright © 2014 John Wiley & Sons, Ltd.     

Core–shell structured CdTe/CdS@SiO2@CdTe@SiO2 composite fluorescent spheres: Synthesis and application for Cd2+ detection

Core–shell structured quantum dot (QD)–silica fluorescent nanoparticles have attracted a great deal of attention due to the excellent optical properties of QDs and the stability of silica. In this study, core–shell structured CdTe/CdS@SiO₂@CdTe@SiO₂ fluorescent nanospheres were synthesized based on the Stöber method using multistep silica encapsulation. The second silica layer on the CdTe QDs maintained the optical stability of nanospheres and decreased adverse influences on the probe during subsequent processing. Red‐emissive CdTe/CdS QDs (630 nm) were used as a built‐in reference signal and green‐emissive CdTe QDs (550 nm) were used as a responding probe. The fluorescence of CdTe QDs was greatly quenched by added S^2?, owing to a S^2?‐induced change in the CdTe QDs surface state in the shell. Upon addition of Cd²⁺ to the S^2?‐quenched CdTe/CdS@SiO₂@CdTe@SiO₂ system, the responding signal at 550 nm was dramatically restored, whereas the emission at 630 nm remained almost unchanged; this response could be used as a ratiometric ‘off–on’ fluorescent probe for the detection of Cd²⁺. The sensing mechanism was suggested to be: the newly formed CdS‐like cluster with a higher band gap facilitated exciton/hole recombination and effectively enhanced the fluorescence of the CdTe QDs. The proposed probe shows a highly sensitive and selective response to Cd²⁺ and has potential application in the detection of Cd²⁺ in environmental or biological samples.     

Aqueous synthesis of highly stable CdTe/ZnS Core/Shell quantum dots for bioimaging

In this work, we report the synthesis, characterization and biological application of highly stable CdTe/ZnS (cadmium tellurite/zinc sulphide) Core/Shell (CS) quantum dots (QDs) capped with mercaptosuccinic acid (MSA). The CS QDs were synthesized using a simple one‐pot aqueous method. The synthesized CdTe/ZnS CS QDs were found to exhibit excellent stability even 100 days after preparation and also showed better photoluminescence quantum yield (PLQY) of about 50% compared with that of only CdTe QDs which was nearly 12%. The formation of the CdTe/ZnS CS was confirmed by high‐resolution transmission electron microscopy (HR‐TEM), and Fourier transform infra‐red (FTIR) and X‐ray diffraction (XRD) analyses. Further, on extending our study towards bioimaging of E. coli cells using the QDs samples, we found that CdTe/ZnS CS QDs showed better results compared with CdTe QDs.     

Characterization and cancer cell targeted imaging properties of human antivascular endothelial growth factor monoclonal antibody conjugated CdTe/ZnS quantum dots

High luminescence quantum yield water‐soluble CdTe/ZnS core/shell quantum dots (QDs) stabilized with thioglycolic acid were synthesized. QDs were chemically coupled to fully humanized antivascular endothelial growth factor₁₆₅ monoclonal antibodies to produce fluorescent probes. These probes can be used to assay the biological affinity of the antibody. The properties of QDs conjugated to an antibody were characterized by ultraviolet and visible spectrophotometry, fluorescent spectrophotometry, sodium dodecyl sulfate–polyacrylamide gel electrophoresis, transmission electron microscopy and fluorescence microscopy. Cell‐targeted imaging was performed in human breast cancer cell lines. The cytotoxicity of bare QDs and fluorescent probes was evaluated in the MCF‐7 cells with an MTT viability assay. The results proved that CdTe/ZnS QD–monoclonal antibody nanoprobes had been successfully prepared with excellent spectral properties in target detections. Surface modification by ZnS shell could mitigate the cytotoxicity of cadmium‐based QDs. The therapeutic effects of antivascular endothelial growth factor antibodies towards cultured human cancer cells were confirmed by MTT assay. Copyright © 2014 John Wiley & Sons, Ltd.     

A simple and sensitive label‐free fluorescence sensing of heparin based on Cdte quantum dots

A rapid, simple and sensitive label‐free fluorescence method was developed for the determination of trace amounts of an important drug, heparin. This new method was based on water‐soluble glutathione‐capped CdTe quantum dots (CdTe QDs) as the luminescent probe. CdTe QDs were prepared according to the published protocol and the sizes of these nanoparticles were verified through transmission electron microscopy (TEM), X‐ray diffraction (XRD) and dynamic light scattering (DLS) with an average particle size of about 7 nm. The fluorescence intensity of glutathione‐capped CdTe QDs increased with increasing heparin concentration. These changes were followed as the analytical signal. Effective variables such as pH, QD concentration and incubation time were optimized. At the optimum conditions, with this optical method, heparin could be measured within the range 10.0–200.0 ng mL^?1 with a low limit of detection, 2.0 ng mL^?1. The constructed fluorescence sensor was also applied successfully for the determination of heparin in human serum. Copyright © 2015 John Wiley & Sons, Ltd.     

Facile synthesis and photophysical characterization of luminescent CdTe quantum dots for Forster resonance energy transfer based immunosensing of staphylococcal enterotoxin B

Aqueous phase synthesis of CdTe quantum dots (QDs) with surface functionalization for bioconjugation remains the best approach for biosensing and bioimaging applications. We present a facile aqueous phase method to prepare CdTe QDs by adjusting precursor and ligand concentrations. CdTe QDs had photoluminescence quantum yield up to ≈33% with a narrow spectral distribution. The powder X‐ray diffraction profile elucidated characteristic broad peaks of zinc blende cubic CdTe nanoparticles with 2.5–3 nm average crystalline size having regular spherical morphology as revealed by transmission electron microscopy. Infra‐red spectroscopy confirmed disappearance of characteristic absorptions for –SH thiols inferring thiol coordinated CdTe nanoparticles. The effective molar concentration of 1 : 2.5 : 0.5 respectively for Cd²⁺/3‐mercaptopropionic acid/HTe^– at pH 9 ± 0.2 resulted in CdTe quantum dots of 2.2–3.06 nm having band gap in the range 2.74–2.26 eV respectively. Later, QD₅₂₃ and QD₆₀₁ were used for monitoring staphylococcal enterotoxin B (SEB; a bacterial superantigen responsible for food poisoning) using Forster resonance energy transfer based two QD fluorescence. QD₅₂₃ and QD₆₀₁ were bioconjugated to anti‐SEB IgY antibody and SEB respectively according to carbodiimide protocol. The mutual affinity between SEB and anti‐SEB antibody was relied upon to obtain efficient energy transfer between respective QDs resulting in fluorescence quenching of QD₅₂₃ and fluorescence enhancement of QD₆₀₁. Presence of SEB in the range 1–0.05 µg varied the rate of fluorescence quenching of QD₅₂₃, thereby demonstrating efficient use of QDs in the Forster resonance energy transfer based immunosensing method by engineering the QD size. Copyright © 2012 John Wiley & Sons, Ltd.     

Conformational and functional effects of MPA‐CdTe quantum dots on SOD: Evaluating the mechanism of oxidative stress induced by quantum dots in the mouse nephrocytes

The application of quantum dots (QDs) is restricted by the biosafety issue. QDs contribute to the adverse effects of organisms probably because of the ability to induce oxidative stress via changing the activity of antioxidant enzyme, for example, superoxide dismutase (SOD). But the underlying molecular mechanisms still remain unclear. This study investigated the harmful effects of oxidative stress induced by mercaptopropionic acid capped CdTe QDs (MPA‐CdTe QDs) on the mouse primary nephrocytes as well as the structure and function of SOD molecule and explored the underlying molecular mechanism. After 24‐hour MPA‐CdTe QD exposure, the activation level of extracellular regulated protein kinase (ERK) signaling pathway and cysteinyl‐directed aspartate‐specific proteases (Caspases) significantly increased, which led to the increasing level of reactive oxygen species (ROS) and cell apoptosis; the group pretreated with ROS scavenger N‐acetyl‐L‐cysteine (NAC) significantly reduced the apoptotic cell percentage, indicating that ROS played a critical role in QD‐induced cytotoxicity. Further molecular experiments showed that the interacting processes between the MPA‐CdTe QDs and SOD were spontaneous which changed the conformation, secondary structure of SOD. The interaction significantly resulted in the tightening of polypeptide chains and the shrinkage of SOD, leading to the inhibition of molecular SOD activity. This study demonstrates the adverse effects of QDs, revealing their potential risk in biomedical applications.     

Influence of the KBr matrix on the luminescence properties of CdTe quantum dots

The investigation of the luminescence properties of CdTe/KBr composites with encapsulated quantum dots (QDs) of different sizes was performed and the influence of the KBr matrix on the luminescence properties of CdTe QDs was studied. Encapsulation of nanoparticles by a solid matrix caused a bathochromic shift in the luminescence peak and the shift value was the larger the smaller the size of the quantum dots. Interband quantum transition theory was used to explain the influence of the matrix on the luminescence properties of the capsulated CdTe QDs. Theoretical calculations showed that the observed QD luminescence peak corresponded to a 1 s–1 s electronic transition, and its low‐energy shift after the transfer of QDs from dielectric water to the KBr matrix was due to a corresponding decrease in the depths of electrons and holes potential wells.     

Involvement of ABC transporters in the efflux and toxicity of MPA‐COOH‐CdTe quantum dots in human breast cancer SK‐BR‐3 cells

This paper aimed to study the possible involvement of adenosine triphosphate‐binding cassette (ABC) transporters in the detoxification of quantum dots (QDs) in human breast carcinoma (SK‐BR‐3) cells. The effects of QD sizes on such interactions were also evaluated. For this purpose, we used monodispersed MPA‐COOH‐CdTe QDs with different diameters (emission length at 560 and 625 nm, named as QD‐560 and QD‐625). Such QDs tended to accumulate in cells and cause significant toxicity. Using specific inhibitors of ABC transporters, the cellular accumulation and toxicity of QDs in SK‐BR‐3 cells were significantly affected. Moreover, treatment of QDs caused concentration‐ and time‐dependent induction of ABC transporters. Furthermore, the induction effects of smaller QDs were found to be greater than larger ones at equivalent concentrations, suggesting a size‐dependent recognition of substrates by ABC transporters. Overall, these results provided important support for the modulation of QDs toxicity by ABC transporters.     

Photoluminescence properties of hybrid SiO2‐coated CdTe/CdSe quantum dots

Hybrid SiO₂‐coated CdTe/CdSe quantum dots (QDs) were prepared using CdTe/CdSe QDs prepared by hydrothermal synthesis. A CdSe interlayer made CdTe/CdSe cores with unique type II heterostructures. The hybrid SiO₂‐coated CdTe/CdSe QDs revealed excellent photoluminescence (PL) properties compared with hybrid SiO₂‐coated CdTe QDs. Because of the existence of spatial separations of carriers in the type II CdTe/CdSe core/shell QDs, the hybrid QDs had a relatively extended PL lifetime and high stability in phosphate‐buffered saline buffer solutions. This is ascribed to the unique components and stable surface state of hybrid SiO₂‐coated CdTe/CdSe QDs. During the stabilization test in phosphate‐buffered saline buffer solutions, both static and dynamic quenching occurred. The quenching mechanism of the hybrid QDs was not suited with the Stern–Volmer equation. However, the relative stable surface of CdTe/CdSe QDs resulted in lower degradation and relative high PL quantum yields compared with hybrid SiO₂‐coated CdTe QDs. As a result, hybrid SiO₂‐coated CdTe/CdSe QDs can be used in bioapplications. Copyright © 2013 John Wiley & Sons, Ltd.     

L‐Cysteine capped CdTe–CdS core–shell quantum dots: preparation,characterization and immuno‐labeling of HeLa cells

Functionalized CdTe–CdS core–shell quantum dots (QDs) were synthesized in aqueous solution via water‐bathing combined hydrothermal method using L‐cysteine (L‐Cys) as a stabilizer. This method possesses both the advantages of water‐bathing and hydrothermal methods for preparing high‐quality QDs with markedly reduced synthesis time, and better stability than a lone hydrothermal method. The QDs were characterized by transmission electronic microscopy and powder X‐ray diffraction and X‐ray photoelectron spectroscopy. The CdTe–CdS QDs with core–shell structure showed both enhanced fluorescence and better photo stability than nude CdTe QDs. After conjugating with antibody rabbit anti‐CEACAM8 (CD67), the as‐prepared l ‐Cys capped CdTe–CdS QDs were successfully used as fluorescent probes for the direct immuno‐labeling and imaging of HeLa cells. It was indicated that this kind of QD would have application potential in bio‐labeling and cell imaging. Copyright © 2009 John Wiley & Sons, Ltd.     

Water‐soluble mercaptoundecanoic acid (MUA)‐coated CdTe quantum dots: one‐step microwave synthesis,characterization and cancer cell imaging

ABSTRACT: In this study, a one‐step approach for aqueous synthesis of highly luminescent semiconductors, CdTe quantum dots (QDs), using long‐chain thiols‐mercaptoundecanoic acid (MUA) as surface ligand, was developed in a microwave irradiation system. The synthetic conditions were systematically investigated. The as‐prepared MUA‐coated QDs were characterized by various spectroscopy techniques, transmission electron microscopy (TEM) and X‐ray powder diffraction (XRD). The experimental results document that MUA‐coated CdTe QDs have small diameter, good stability, high luminescence and long lifetime. Particularly, it was confirmed, using fluorescence correlation spectroscopy (FCS) that, compared with other ligand, MUA formed a thicker ligand layer on the QD surfaces, which will help their stability and conjugation with biomolecules. Furthermore, MUA‐coated QDs were successfully used for HeLa cell imaging. Copyright © 2011 John Wiley & Sons, Ltd.     

Studying nanotoxic effects of CdTe quantum dots in Trypanosoma cruzi

Semiconductor nanoparticles, such as quantum dots (QDs), were used to carry out experiments in vivo and ex vivo with Trypanosoma cruzi. However, questions have been raised regarding the nanotoxicity of QDs in living cells, microorganisms, tissues and whole animals. The objective of this paper was to conduct a QD nanotoxicity study on living T. cruzi protozoa using analytical methods. This was accomplished using in vitro experiments to test the interference of the QDs on parasite development, morphology and viability. Our results show that after 72 h, a 200 μM cadmium telluride (CdTe) QD solution induced important morphological alterations in T. cruzi, such as DNA damage, plasma membrane blebbing and mitochondrial swelling. Flow cytometry assays showed no damage to the plasma membrane when incubated with 200 μM CdTe QDs for up to 72 h (propidium iodide cells), giving no evidence of classical necrosis. Parasites incubated with 2 μM CdTe QDs still proliferated after seven days. In summary, a low concentration of CdTe QDs (2 μM) is optimal for bioimaging, whereas a high concentration (200 μM CdTe) could be toxic to cells. Taken together, our data indicate that 2 μM QD can be used for the successful long-term study of the parasite-vector interaction in real time.     

CdTe/ZnS quantum dots as fluorescent probes for ammonium determination

Novel CdTe/ZnS quantum dot (QD) probes based on the quenching effect were proposed for the simple, rapid, and specific determination of ammonium in aqueous solutions. The QDs were modified using 3‐mercaptopropionic acid, and the fluorescence responses of the CdTe/ZnS QD probes to ammonium were detected through regularity quenching. The quenching levels of the CdTe/ZnS QDs and ammonium concentration showed a good linear relationship between 4.0 × 10^?6 and 5.0 × 10^?4 mol/L; the detection limit was 3.0 × 10^?7 mol/L. Ammonium contents in synthetic explosion soil samples were measured to determine the practical applications of the QD probes and a probable quenching mechanism was described. Copyright © 2015 John Wiley & Sons, Ltd.     

One‐pot synthesis and characterization CdTe:Zn2+ quantum dots and its molecular interaction with calf thymus DNA

Tremendous research efforts have been dedicated to fabricating high‐quality Zn‐doped CdTe quantum dots (QDs) for any potential biomedical applications. In particular, the correlation of issues regarding how QDs interact with DNA is of greatest importance. Herein, a pH‐responsive study of the interactions between CdTe:Zn²⁺ quantum dots with 4 different sizes and calf thymus DNA (ctDNA) was conducted using multispectroscopic techniques and electrochemical investigation. Fluorescence studies revealed that this interaction process is predominantly a static process and groove binding was the main binding mode for CdTe:Zn²⁺ QDs to ctDNA. The calculated negative values of enthalpy (?45.06 kJ mol^?1) and entropy (?133.62 J mol^?1 K^?1) with temperature changes indicated that the hydrogen bonds and van der Waals interactions played major roles in the reaction. Furthermore, circular dichroism spectroscopy and Fourier transform infrared spectrometry analyses indicate that the normal conformation of ctDNA is discombobulated by CdTe:Zn²⁺ QDs. In addition, the electrochemical behavior of the affinity of CdTe:Zn²⁺ QDs for ctDNA agreed well with the results obtained from fluorescence experiments. This study might be meaningful for understanding the molecular binding mechanism of QDs for DNA and provides a basis for QD‐labeled systems.     

Labeling of BSA and imaging of mouse T‐lymphocyte as well as mouse spleen tissue by l‐glutathione capped CdTe quantum dots

l ‐glutathione capped highly fluorescent CdTe quantum dots (QDs) were prepared by an aqueous approach and used as fluorescent labels to link albumin bovine serum (BSA) and rat anti‐mouse CD4, which was expressed on mouse T‐lymphocyte and mouse spleen tissue. The sharp and narrow emission peaks showed that the as‐prepared QDs have desirable dispersibility, uniformity and good fluorescence properties. Both CdTe–BSA and CdTe–CD4 conjugates showed an enhancement of fluorescence intensity over that of bare CdTe QDs. The experimental result of gel electrophoresis confirmed the successful conjugation of CdTe–BSA and CdTe–CD4. The fluorescent microscopic images of CdTe–CD4 labeled mouse T‐lymphocyte cells and mouse spleen tissue were compared with that obtained from fluorescein isothiocyanate labeling. It was demonstrated that the CdTe QDs‐based probe exhibited much better photostability and fluorescence intensity than fluorescein isothiocyanate, showing a good application potential in the immuno‐labeling of cells and tissues. Copyright © 2009 John Wiley & Sons, Ltd.     

One‐Pot Gram‐Scale,Eco‐Friendly,and Cost‐Effective Synthesis of CuGaS2/ZnS Nanocrystals as Efficient UV‐Harvesting Down‐Converter for Photovoltaics

It is presented for the first time nontoxic CuGaS₂/ZnS quantum dots (QDs) with free‐self‐reabsorption losses and large Stokes shift (>190 nm) synthesized on an industrially gram‐scale as an alternative for Cd‐based energy‐downshift (EDS)‐QD layers. The QDs exhibit a typical EDS that absorbs only UV light (<407 nm) and emits the whole range of visible light (400–800 nm) with a high photoluminescence‐quantum yield of ≈76%. The straightforward application of these EDS‐QDs on the front surface of a monocrystalline p‐type silicon solar cell significantly enhances the short‐circuit current density by ≈1.64 mA cm^?2 (+4.20%); thereby, improving the power‐conversion‐efficiency by ≈4.11%. The significant improvement in the external quantum efficiency increases by ≈35.7% and that in the surface reflectance decreases by ≈14.1% in the UV region (300–450 nm) clearly manifest the photovoltaic enhancement. Such promising results together with the simple (one‐pot core/shell synthesis), cost‐effective (reduction in a bill of material–system by ≈2.62%), and scalable (2000 mL three‐neck flask, 11 g of QDs) preparation process might encourage the manufacturers of solar cells and other optoelectronic applications to apply these EDS‐QDs to different broader eco‐friendly applications.     

Quantum dots as nano plug-in's for efficient NADH resonance energy routing

The routing of fluorescent signals from NADH to quantum dots (QDs) has been a subject of extensive research for FRET based applications. In the present study, the spectral cross talk of NAD(+)/NADH with QDs was used to monitor the reaction of NAD(+)-dependent dehydrogenase enzyme. CdTe QD may undergo dipolar interaction with NADH as a result of broad spectral absorption due to multiple excitonic states resulting from quantum confinement effects. Thus, non-radiative energy transfer can take place from NADH to CdTe QD enhancing QDs fluorescence. Energy routing assay of NADH-QD was applied for detection of formaldehyde as a model analyte in the range 1000-0.01 ng/mL by the proposed technique. We observed proportionate quenching of CdTe QD fluorescence by NAD(+) and enhancement in the presence of NADH formed by various concentrations of enzyme (0.028-0.4 U). Hence, it was possible to detect formaldehyde in the range 1000-0.01 ng/mL with a limit of detection (LOD) at 0.01 ng/mL and regression coefficient R(2)=0.9982. Therefore, a unique optical sensor was developed for the detection of the formaldehyde in sensitive level based on the above mechanism. This method can be used to follow the activity of NAD(+)-dependent enzymes and detection of dehydrogenases in general.