Exhibition of Förster resonance energy transfer from CdSe/ZnS quantum dots to zinc porphyrazine studied in solution

Transient absorption study on fluorescence quenching of InP/ZnS quantum dots by MXene

Fluorescence quenching due to energy transfer from InP/ZnS quantum dots (QDs) to Ti3C2Tx MXene materials was investigated by the transient absorption spectroscopy. During the fluorescence quenching, the photo-induced absorption feature in the transient spectrum was blue-shifted due to the higher photon energy required for the upward transition. The lifetime of stimulated emission was gradually extended from 0.86 μs to 2.28 μs with increasing Ti3C2Tx MXene. The fluorescence quenching of QDs can be quantitatively characterized by analyzing the lifetime of the stimulated emission feature in the transient absorption spectrum, which was used as a Ti3C2Tx MXene detection with the sensitivity of 6.63 mL/mg. The results of this study provide the basis for the design of optical sensors.

Fӧrster resonance energy transfer (FRET) between CdSe quantum dots and ABA phosphorus(V) corroles

In this article, highly fluorescent phosphorus(V) corrole was synthesised which was then combined with CdSe quantum dots (QDs) in order to study Förster resonance energy transfer (FRET) mechanism between CdSe QDs (donor) and phosphorus corrole (acceptor). Spectral overlap between QD's emission profile and corrole's absorption profile was found to be significant enough to result into Förster resonance energy transfer (FRET). The UV-vis spectrum experienced increase in the absorption bands on addition of phosphorus corrole to CdSe QDs suggesting QD-corrole conjugation. In the steady state fluorescence measurements, emission spectrum observed quenching in the fluorescence intensity of prepared CdSe QDs on addition of phosphorus corrole. Likewise, in case of time-resolved fluorescence measurements it was noticed that the CdSe QD's lifetime was greatly quenched by the presence of a corrole acceptor. Stern-Volmer plot was made to show quenching in this case was dynamic in nature. Based on the results of UV-vis, steady state and time-resolved fluorescence measurements the plausible mechanism behind such observations is considered to be FRET.

Dualistic Fluorescence as Well as Portable Smartphone-Assisted RGB-Relied Sensing Assay for the Ultra-Sensitive Determination of Pendimethalin in Food and Water Samples by AIEE Active Organic Probes

Herein, a peculiar fluorometric as well as smartphone-assisted RGB-relied sensing assay is introduced for determining pendimethalin (PDM) herbicide contents (in parts per trillion level) based on the anthracene-incorporated pyrimidinone/thione probes (S1 to S4). These compounds offered a unique and impressive aggregation-induced emission enhancement (AIEE) behavior by aggregation in H₂O-dimethylformamide medium. Furthermore, these AIEE active compounds were found to display superior selectivity and extraordinary sensitivity for PDM detection via fluorescence quenching response. The extent of quenching degree was found to be linearly varied with the PDM concentration ranging from 0 to 20 nM, with a lower limit of detection of 367.8 pM (103.4 ppt) by S3 nanoaggregates. The detailed investigation revealed that such a high sensitivity of the designed sensor toward PDM is attributable to the existence of dual "photoinduced charge transfer and Förster resonance energy transfer process mechanisms". The Stern-Volmer plots, Job's plot, Benesi-Hildebrand plot, and ¹H NMR titrations as well indicated the existence of substantial interactions between the sensor and PDM. The conducted selectivity tests provided distinguishable selectivity for PDM detection over various other insecticides/pesticides as well as other structural nitro analogues. Additionally, the presented sensing assay was also applied to quantify the PDM residues in spiked food (vegetables, fruits, and grains) and water samples. In addition, the sensor-coated fluorescent paper test strips were also fabricated for on-site detection of PDM. The applicability of smartphone-relied RGB analysis significantly streamlined the operation process, speeds up the detection procedure, and also offered a novel methodology for real-time analysis of PDM in real samples.

Spectroscopic investigation on the binding interactions between graphene quantum dots and carbonic anhydrase

As a new member of the nanomaterials family, ultrasmall graphene quantum dots (GQDs) have shown broad application prospects in the field of biomedicine, but the analysis of their biological effects at the molecular level is yet limited. Herein, carbonic anhydrase (CA) was selected as a model protein to assess the interactions between GQDs and biomacromolecules. A range of spectroscopic techniques were employed to systematically investigate the binding interactions between GQDs and CA and the catalytic function of CA in the presence of GQDs was evaluated. Experimental results showed that GQDs could quench the intrinsic fluorescence of CA and the concentration dependent quenching efficiency exhibited an obvious deviation from the linear plot, indicating a static binding mode. Further investigation suggested that van der Waal interactions and hydrogen bonding were the main driving forces. Additionally, circular dichroism measurement showed that the binding of GQDs induced slight conformational changes of CA. The catalytic capability assessment proved that these binding interactions resulted in the reduction of the biological functions of CA. This comprehensive study provided important insight into the interaction of GQDs with biomacromolecules, which would be crucial for the further applications of GQDs and other nanomaterials in the biomedical field.

Tandem Förster resonance energy transfer induced visual ratiometric fluorescence sensing of tetracyclines based on zeolitic imidazolate framework-8 incorporated with carbon dots and safranine T

With increasing TC concentration, tandem FRET1 from CDs to TC, then FRET2 from TC to safranine T were occurred. TC could be easily recognized by naked eye. Besides, we could perform on-site detection of TC with the help of a mobile phone.

Fluorescence biosensor based on silicon quantum dots and 5,5'-dithiobis-(2-nitrobenzoic acid) for thiols in living cells

An efficient and stable fluorescent sensor is described for the detection and imaging of thiols. It is making use of silicon quantum dots (SiQDs) which can be rapidly prepared. They were characterized by transmission electron microscopy, X-ray power diffraction, Fourier transform infrared spectroscopy, X-ray photoelectron spectrometry. The SiQDs have an absorption maximum at 300 nm and displayed blue-green fluorescence with excitation/emission maxima at 410/480 nm. A mixture of SiQDs and 5,5'-dithiobis-(2-nitrobenzoic acid) (DTNB) exhibits strong fluorescence emission which however is quenched within 30 s of incubation with thiols. This is assumed to be due to an inner filter effect caused by the reaction of DTNB and thiols. The following thiols were tested: cysteine, homocysteine, and glutathione. The sensor has a linear response in the 3-100 μM thiol concentration range, and the LODs are between 0.80 and 0.96 μM. The sensor displays low cytotoxicity and was applied to fluorescence imaging of MCF-7 cells and Hela cells where it demonstrated excellent biocompatibility.

Carbon quantum dots as fluorescence sensors for label-free detection of folic acid in biological samples

Carbon quantum dots (CQDs) have been fabricated by a facile single-step pyrolysis method from citric acid and ethylene imine polymer. When excited at 359 nm, CQDs show intense blue fluorescence at 434 nm. The fluorescence can be effectively quenched by folic acid (FA), which is attributed to the combination of static quenching and inner filter effect. Thus, the CQDs are developed as an efficient fluorescent sensing platform for label-free sensitive and selective detection of FA. Key parameters influencing the detection were investigated, such as incubation time, salt concentration, selectivity and potential interferences. Under the optimal conditions, a good linearity was observed for the emission intensity against 1.14-47.57 μM with a correlation coefficient of 0.99. The limit of detection was found to be 0.38 μM. The practical application of the sensing system was demonstrated by analyzing FA in human urine samples. The sample recoveries fell in the range of 82.0%-113.1% with RSDs ≤ 10.9%, which presented its reliable and feasible application in real samples.

A simulation study on the influence of energy migration and relative interaction strengths of homo- and hetero-FRET on the net FRET efficiency

Förster resonance energy transfer (FRET) is a powerful method for probing biomolecular conformations and dynamics in bulk as well as at a single-molecule level. FRET utilizes non-radiative mechanisms to transfer energy between fluorophores, donor and acceptor when placed in close proximity. The FRET efficiency has a strong distance dependence and serves as a direct read-out for molecular interaction. In case of a significant overlap of donor emission and absorption spectra, the excited state energy can be exchanged between the identical donors in close proximity, which eventually migrates back and forth until it gets dissipated. This form of energy transfer is called energy migration or homo-FRET. Here, we have simulated FRET efficiency by considering the donor-donor interaction strength (ξ_DD) and donor-acceptor interaction strength (ξ_DA) under conditions of non-uniform distribution of molecules. Our earlier studies indicate that energy migration modulate the FRET efficiency for various values of ξ_DD and ξ_DA. We, therefore, determined the limiting values of acceptor concentration (C_LA) that will allow the determination of FRET efficiency in the absence and presence of energy migration. Taken together, our study optimizes the conditions for meaningful FRET efficiency for a given FRET pair for better reporting of molecular interactions.

CdTe QDs based fluorescent sensor for the determination of gallic acid in tea

A new fluorescent light switch method, which based on N‑acetyl‑l‑cysteine capped CdTe QDs (NALC-CdTe QDs), was developed for the detection of gallic acid (GA). The QDs possess a fluorescence emission wavelength at 520nm and with symmetric fluorescence. When KMnO₄ is added, the high fluorescence of QDs could be effectively quenched for the electron transfer process between KMnO₄ and QDs. But with the addition of GA, the fluorescence of KMnO₄-QDs system could recover for the reason that redox reaction of GA and KMnO₄. Therefore, a fluorescent light switch method could be used for GA with a detection range of 0.6-12.6μg·mL^-1 and a detection limit of 0.56ng·mL^-1. Furthermore, the feasibility of the proposed fluorescence biosensor in tea was also studied and satisfactory results were obtained.

Light-harvesting porphyrazines to enable intramolecular singlet fission

A porphyrazine featuring complementary absorption to a pentacene dimer was chosen to fill the absorption gap of the latter in the range of 450 to 600 nm to realize panchromatic absorption through the visible region out to ca. 700 nm. Of even greater relevance is the quantitative intramolecular Förster resonance energy transfer (i-FRET) to funnel energy to the pentacene moieties, where efficient intramolecular singlet fission (i-SF) converts the singlet excited state into the corresponding triplet excited states. Remarkably, the triplet quantum yield either via direct excitation or via indirect i-FRET is up to 200% ± 20% in polar solvents.

Sulfur and phosphorus co-doped graphene quantum dots for fluorescent monitoring of nitrite in pickles

Doping graphene quantum dots (GQDs) with heteroatoms can change their band gap and electronic density, thus enhancing their fluorescence quantum yield (QY). In this work, we for the first time reported a nontoxic, rapid, and one-pot hydrothermal method to synthesize sulfur and phosphorus co-doped GQDs (S, P-GQDs). Citric acid was functioned as a carbon source, whereas sodium phytate and anhydrous sodium sulfate are used as the P and S sources, respectively, in this bottom-up synthesis. The resulting S, P-GQDs exhibit high heteroatomic doping ratios of 9.66 at.% for S and 3.34 at.% for P, and higher QY than those obtained from monoatomic doped GQDs. Additionally, the as-prepared S, P-GQDs exhibit excitation-dependent behavior, pH sensitivity between 8.0 and 13.0, high tolerance of ionic strength. More importantly, the as-synthesized S, P-GQDs show a sensitive and selective behavior for sensing nitrite (NO₂^-) in the concentration range of 0.7-9 μmol/L, and the detection limit was as low as 0.3 μmol/L. Additionally, the S, P-GQDs was successfully used in detecting NO₂^- in pickled foods, showing their promise for potential applications in realistic analysis.

ZnCdSe-CdTe quantum dots: A "turn-off" fluorescent probe for the detection of multiple adulterants in an herbal honey

To enhance the power of untargeted detection, a "turn-off" fluorescent probe with double quantum dots (QDs) was developed and coupled with chemometrics for rapid detection of multiple adulterants in an herbal (Rhus chinensis Mill., RCM) honey. The double water-soluble ZnCdSe-CdTe QDs have two separate and strong fluorescent peaks, which can be quenched by honey and extraneous adulterants with varying degrees. Class models of pure RCM honey samples collected from 6 different producing areas (n = 122) were developed using one-class partial least squares (OCPLS). Four extraneous adulterants, including glucose syrup, sucrose syrup, fructose syrup, and glucose-fructose syrup were added to pure honey samples at the levels of 0.5% to 10% (w/w). As a result, the OCPLS model using the second-order derivative (D2) spectra could detect 1.0% (w/w) of different syrups in RCM honey, with a sensitivity of 0.949. The double water-soluble QDs, which can be adjusted for analysis of other water-soluble food samples, has largely extended the capability of traditional fluorescence and will provide a potentially more sensitive and specific analysis method for food frauds.

Evaluation of the effects of mild heat in bovine milk by time resolved fluorescence

Heat treatment of milk and dairy products are indispensable for the dairy industry. This thermal processing intends to extend shelf life, improve quality of the milk and minimize the health risks associated with milk and dairy products. The use of time-resolved fluorescence techniques to identify conformation and structure changes ok milk fat and proteins could help understand the temperature effects in bovine milk. This study aimed to use fluorescence lifetimes to evaluate the effects of heating fresh cow milk up to 85 °C. We observed different tendencies for fluorescence lifetimes submitted to different heating temperatures. The longer lifetime values decreased for temperatures higher than room temperature until it reached a minimum value near 40 °C and it slowly increased again for temperatures higher than 40 °C, indicating two distinct processes. These results indicate that time-resolved fluorescence can assist on the analysis of heating effects in fluid milk.

A ratiometric fluorescence and light scattering sensing platform based on Cu-doped carbon dots for tryptophan and Fe(III)

A new Cu-doped carbon dots (Cu-CDs) were synthesized rapidly and simply via one-step thermolysis of Na₂[Cu(EDTA)] and ascorbic acid (AA) at 250°C for 2h with a high quantum yield of 9.8%. The Cu-CDs exhibits two signals of fluorescence at 396nm and second-order scattering (SOS) at 617nm under a single excitation wavelength of 308nm, and can be obviously enhanced by tryptophan (Trp) or Fe(III) leading to the ratiometric fluorescence and SOS response with a good linear wider range of 0.5-250μM and 0.1-50μM, respectively. This sensing system exhibits good selectivity and sensitivity toward Trp and Fe(III) over other analytes with a low detection limit of 275nM and 46nM, respectively. Furthermore, the proposed sensing system displays a prospective application for quantitative assay of Trp and Fe(III) in practical sample.