Folic Acid as a Bimodal Optical Probe for the Detection of TNT.

Rapid and onsite detection of nitroaromatic explosive 2,4,6-trinitrotoluene (TNT) is very crucial for the safety and security of human life as well as for the environment. In this present work, we demonstrate the feasibility for employing Folic Acid (FA) as a fluorescent as well as a colorimetric probe for the detection of TNT. This probe was synthesized by a simple one-step process. The developed probe shows an emission maximum at 490 nm upon excitation at 420 nm. On adding TNT, the fluorescence of the FA probe is quenched. Also, it shows a good selectivity towards TNT over other similar organic compounds such as 4-nitrophenol (4-NP), 2,4-dinitrophenol (2,4-DNP) and picric acid (PA). The limit of detection (LoD) of TNT was found to be 1.9398 µM. Colorimetric detection was conducted and paper strip assay was developed for the practical applications.

Investigation of Heavy Atom Effect on Fluorescence of Carbon Dots: NCDs and S,N-CDs.

The present work is an attempt to investigate the heavy atom effect imparted by halide ions, especially iodide (I-) ions on the fluorescence behavior of carbon dots (CDs). Here two different types of CDs viz. nitrogen doped carbon dots (NCDs) from Citric Acid (CA) & Urea and Sulfur and nitrogen co-doped carbon dots (S,N-CDs) from CA & L-Cysteine were synthesized and the fluorescence of both CDs were quenched by heavy atom effect on adding potassium iodide (KI). The large difference in average lifetime was obtained in time-correlated single-photon counting (TCSPC) analysis suggests the dynamic quenching and heavy atom effect. Graphical abstract.

Dopamine-induced photoluminescence quenching of bovine serum albumin-capped manganese-doped zinc sulphide quantum dots.

The direct detection of dopamine (DA) in human body fluids is a great challenge for medical diagnostics of neurological disorders like Parkinson's disease, Alzheimer's disease, senile dementia, and schizophrenia. In this work, a simple and turn off luminescence sensing of DA based on bovine serum albumin (BSA)-capped manganese-doped zinc sulphide quantum dots (Mn:ZnS/BSA QDs) is developed. The Mn:ZnS/BSA QDs were synthesized by a chemical co-precipitation method. Due to the special interaction of DA with BSA and metal ions, Mn:ZnS/BSA QDs can serve as an effective sensing platform for DA. The luminescence of Mn:ZnS/BSA QDs decreased linearly with increasing concentration of DA in the range from 6.6 to 50.6 nM. The limit of detection is 2.02 nM. The driving force for the luminescence quenching is partly provided by ground-state complex formation of QDs with DA. The photo-induced electron transfer from the conduction band of QDs to oxidized dopamine (quinone) also favors quenching. The Mn:ZnS/BSA QDs are barely interfered with by other competing biomolecules except catecholamine neurotransmitter like epinephrine. Moreover, this method is used in the analysis of DA-spiked human serum and human urine samples and good recovery percentages are found. To assess the utility of the developed sensor, paper strip assay was also successfully conducted. Graphical abstract.

Amplified luminescence quenching effect upon binding of nitrogen doped carbon nanodots to transition metal ions.

There is a significant drive to identify a unified emission mechanism hidden behind carbon nanodots (CDs) to attain reliable control over their photoluminescence properties. This issue is addressed here by investigating the fluorescence response of citric acid and urea-based nitrogen doped carbon nanodots (NCDs) towards transition metal ions in solutions of different polarities/viscosities/hydrogen bonding strengths. The photoluminescence from NCDs upon excitation at 400 nm is quenched by metal ions such as chromium(vi), ruthenium(iii) and iron(iii) in two different polar solvents, protic water and aprotic dimethylsulphoxide (DMSO). This amplified luminescence quenching in polar solutions showed significant static quenching contributions. The quenching phenomenon highly depends on the excitation wavelength and solvent environment. The fluorescence quenching sequence reveals that pyridinic nitrogen-bases have a dominant influence on J-like emissive aggregates of NCDs. Similarly, oxygen-containing functional groups play a significant role in constructing H-aggregates of NCDs. The most intense emission is contributed by the J-like assembly of H-aggregates.

Solvent Effects: A Signature of J- and H-Aggregate of Carbon Nanodots in Polar Solvents.

The secret behind excitation-dependent/-independent photoluminescence of carbon nanodots (CDs) is not yet revealed completely. To address this issue, a detailed investigation on solvent polarity-dependent optical properties of citric acid-urea co-derived nitrogen-doped carbon nanodots (NCDs) was carried out. The interpretation on UV-visible spectral data reveals the presence of H-aggregates formed through hydrogen bonding. In addition, dipole-dipole interaction-mediated J-aggregates are clearly evident. The broad and intense excitation band of NCDs is mostly contributed by highly emissive J-like self-assembly of H-aggregates in polar solvents. Time-resolved fluorescence spectra of NCDs show triexponential decay kinetics. The three lifetime components correspond to long-lived H-aggregates, short-lived J-aggregates, and JH-aggregates of intermediate lifetime. Moreover, fluorescence of NCD is influenced by concentration and storage time. Accordingly, mismatch in spectral shapes of excitation and absorption spectra of NCD can be successfully correlated to aggregate species of NCDs that exist even in very dilute solutions.

Potassium triiodide-quenched gold nanocluster as a fluorescent turn-on probe for sensing cysteine/homocysteine in human serum.

A fluorescent sensing platform using KI3-quenched bovine serum albumin stabilized gold nanoclusters has been designed and used as a fluorescent probe for the turn-on detection of homocysteine/cysteine (Cys/Hcy). The fluorescence of gold nanoclusters was quenched by iodine. The fluorescence of quenched gold nanoclusters was effectively switched on by Cys/Hcy devoid of the interference of glutathione. The transmission electron microscopy image, X-ray photoelectron spectroscopy analysis, time-correlated single photon counting analysis, and dynamic light scattering data confirmed the aggregation-induced quenching of fluorescence of gold nanoclusters by iodine. The turn-on response of Cys/Hcy shows two linear ranges from 0.0057 to 5 µM and from 8 to 25 µM, with a limit of detection of 9 nM for cysteine and 12 nM for homocysteine. Real samples were analyzed to monitor Cys/Hcy added to human serum. The fluorescence turn-on response of the probe on a paper strip in the presence of Cys/Hcy was studied. Graphical abstract ᅟ.

Reversible fluorescence modulation of BSA stabilised copper nanoclusters for the selective detection of protamine and heparin.

Protamine and heparin are the most important polyionic drugs used during surgeries and extracorporeal therapies. In this article, a selective and sensitive fluorescence method for the detection of both protamine and heparin was developed by using bovine serum albumin stabilised copper nanoclusters. Blue emitting fluorescent copper nanoclusters were synthesized in aqueous solution using bovine serum albumin as a capping agent and a reducing agent. A one pot microwave assisted method was adopted to synthesize fluorescent copper nanoclusters showing emission at 410 nm upon excitation at 330 nm. The fluorescence of copper nanoclusters was found to be enhanced after the addition of protamine and the limit of detection obtained is 0.12 ng mL-1. The significant enhancement in fluorescence can be attributed to the electrostatic interactions between the copper nanocluster and protamine. In contrast, the enhanced fluorescence intensity of the copper nanocluster with protamine added was decreased after the addition of heparin, and the copper nanocluster regained its original fluorescence intensity. This can be attributed to the strong interaction of protamine with heparin and the limit of detection was calculated as 0.0406 ng mL-1. The selectivity and sensitivity of the sensor for both protamine and heparin were also determined in the presence of potentially co-existing biomolecules, cations, and anions and satisfactory results were obtained. Additionally the validity of the proposed protamine and heparin sensor was attested in real sample matrices such as human urine samples and human blood serum samples. The results exhibited that the recovery percentage of protamine and heparin reached 98-99% and 92-99% in urine samples and 97-99% in serum samples.

Fluorometric determination of morphine via its effect on the quenching of fluorescein by gold nanoparticles through a surface energy transfer process.

A method is described for sensitive and selective fluorometric determination of morphine. It is based on the effect of morphine on quenching of the fluorescence of fluorescein by gold nanoparticles (AuNPs) via surface energy transfer. When fluorescein is added to solutions of colloidal AuNPs, its fluorescence becomes quenched due to nanometal surface energy transfer (NSET) because the absorption of AuNPs strongly overlaps the emission spectrum of fluorescein. In the presence of morphine, which contains both a tertiary nitrogen ring atom and a phenolic hydroxy group, it will coordinate to the AuNPs, and this causes recovery of fluorescence. The presence of a tertiary nitrogen ring atom and a phenolic hydroxy group (both required for the effect to occur) in morphine make the probe highly selective and sensitive for morphine. A paper strip assay also was developed by utilizing this detection scheme. The turn-on fluorescent probe was successfully applied to the determination of morphine in spiked serum and urine samples. The method has a 53 pM limit of detection. The paper strip was applied to the determination of morphine in sweat, urine and other biological fluids. It is perceived to be useful for early detection of drug abuse by adolescent. Graphical abstract Schematic of the mechanism of fluorescence turn on detection of morphine using Au NPs (gold nanoparticles) acting asquencher of the fluorescence of fluorescein.

Fluorescence turn on detection of bilirubin using Fe (III) modulated BSA stabilized copper nanocluster; A mechanistic perception.

Hyperbilirubinemia is the condition when bilirubin exceeds normal concentration in body (19.80 mg/mL in newborns and 1.19 × 10-2 mg/mL in adults). Bilirubin encephalopathy in newborns may cause irreversible neurological disorders. Current methods for detection of bilirubin suffer from compromising accuracy. In the present work, bovine serum albumin stabilized copper nanocluster (BSA-CuNCs) was synthesized via a one pot microwave assisted method as a turn on detector for bilirubin. The synthesized BSA-CuNCs having size less than 4 nm, exhibited bright blue emission at 405 nm. Interestingly, no observable change in fluorescence emission was noticed over a wide pH range (1-11) or at high ionic conditions. However, the interaction of Fe3+ with BSA-CuNCs induces quenching of fluorescence. Moreover, the fluorescence can be regained by the addition of bilirubin over other possible coexisting biomolecules. A good linearity was observed for BSA-CuNCs based turn on probe with a Limit of Detection (LoD) 6.62 nM. Furthermore, real sample analyses were carried out with human serum and urine which showed good recovery percentage.

S,N-doped carbon dots as a fluorescent probe for bilirubin.

Carbon dots doped with sulfur and nitrogen (S,N-CDs) were utilised to design a paper-stripe based fluorescent probe for the detection of bilirubin. The S,N-CDs were synthesized through a microwave assisted route by using citric acid as carbon source and L-cysteine as a source of nitrogen and sulfur. The S,N-CDs exhibit bright blue fluorescence emission with a peak at 452 nm. Fluorescence is quenched by Fe(III) but selectively restored by bilirubin. The quenched fluorescent probe exhibit significant selectivity and sensitivity for bilirubin in the 0.2 nM to 2 nM concentration range, with a 0.12 nM detection limit. The method was applied to the determination of bilirubin in spiked human serum and urine samples. The method was used to design a paper based test stripe as a point of care device for visual bilirubin detection. Graphical abstract Schematic representation of sulphur and nitrogen doped carbon dots whose fluorescence is quenched by Fe(III) and turned on by bilirubin. Photograph of the corresponding system under day light and UV shows the feasibility of the phenomenon. The applicability of the assay was further extended by impregnating the probe on a filter paper.