A portable and intelligent logic detector for simultaneous and in-situ detection of Al3+ and fluoride in groundwater

To develop a convenient and intelligent detector for simultaneous and in-situ detection of Al³⁺ and F^- in groundwater, a novel organic probe called RBP has been prepared. With the increase of Al³⁺, RBP showed a significant fluorescence enhancement at 588 nm, and the detection limit was 0.130 mg/L. After combining with fluorescent internal standard CDs, the fluorescence of RBP-Al-CDs at 588 nm was quenched due to the replace of F^- for Al³⁺, while the CDs at 460 nm remained unchanged, and the detection limit was 0.0186 mg/L. For convenient and intelligent detection, an RBP-based logic detector has been developed for simultaneous detection of Al³⁺ and F^-. Within the ultra-trace, low concentration, and high concentration range of Al³⁺ and F^-, the logic detector can achieve rapid feedback on their concentration levels ("U", "L" and "H") through different output modes of the signal lamps. The development of logical detector is of great significance for studying the in-situ chemical behavior of Al³⁺ and F^- and for daily household detection.

A turn-on fluorescent nano-probe base on methanobactin-AuNPs for simple and efficient detection of nitrite

Nitrite ions are important markers threatening humans and environmental security. A highly selective method for rapid detection of nitrite needs to be developed. Herein, a novel and rapid fluorescence method for nitrite determination is established on the basis of diazotization-coupling reaction of methanobactin (Mb) extracted by Methylosinus trichosporium OB3b with nitrite on the fluorescence. In the presence of gold nanoparticles (AuNPs), the fluorescence of AuNPs was strongly quenched by the Mb because the sulfhydryl or amino structures on the surface of Mb could be bound to the surface of AuNPs by forming Au-S or Au-N bonds. Upon addition of nitrite, the Mb easily reacts with nitrite to form azo products in the acidic medium. Then, with the increase of nitrite concentration, the Mb-AuNPs fluorescence was gradually recovered, realizing the turn-on fluorescence sensing of nitrite. Under optimal conditions, the proposed method has a good linear relationship with nitrite concentration in the range of 0-8.0 μM and 8.0-50.0 μM, and the detection limit is 16.21 nM. In addition, satisfactory results were obtained for nitrite analysis using milk, ham sausage and leaf mustard as real samples, which demonstrated that the method as-developed would have great practical application prospects.

A versatile logic detector and fluorescent film based on Eu-based MOF for swift detection of formaldehyde in solutions and gas phase

Due to the huge threat of formaldehyde (FA) on human beings, the development of chemical sensors for swift detection of FA in solutions and gas phase is highly anticipated. In this paper, a versatile logic detector and a portable fluorescent film based on small-scaled Eu-based MOF were applied successfully to detect FA in solutions and gas phase, respectively. For FA in aqueous solution, the design of logic detector will efficiently identify FA in different concentration ranges: when the FA concentration are 0-500 ppb, 500-1000 ppb and >1000 ppb, the output signals of logic detector are the concentration level of FA ("L", "H" and "VH"), and accompanied by red, purple and blue signal lamps to remind, respectively. For FA in the air, the color of rigid film sensor will gradually change from red to blue with the increase of FA under UV lamp, and the detection limit of gaseous FA is 11.8 ppb. Through the preparation of logic devices and fluorescent films, Eu-based MOF realized swift detection of FA in solutions and gas phase, which will be very helpful to improve the human response level to FA from different emission sources.

The synthesis of a chemically reactive and polymeric luminescent gel

In the past, chemically reactive polymeric interfaces have been considered to be of potential interest for developing functional materials for a wide range of practical applications. Furthermore, the rational incorporation of luminescence properties into such chemically reactive interfaces could provide a basis for extending the horizon of their prospective utility. In this report, a simple catalyst-free chemical approach is introduced to develop a chemically reactive and optically active polymeric gel. Branched-polyethyleneimine (BPEI)-derived, inherently luminescent carbon dots (BPEI-CDs) were covalently crosslinked with pentaacrylate (5Acl) through a 1,4-conjugate addition reaction under ambient conditions. The synthesized polymeric gel was milky white under visible light; however, it displayed fluorescence under UV light. Additionally, the residual acrylate groups in the synthesized fluorescent gel allowed its chemical functionality to be tailored through facile, robust 1,4-conjugate addition reactions with primary-amine-containing small molecules under ambient conditions. The chemical reactivity of the luminescent gel was further employed for a proof-of-concept demonstration of portable and parallel 'ON'/'OFF' toxic chemical sensing (namely, the sensing of nitrite ions as a model analyte). First, the chemically reactive luminescent gel derived from BPEI-CDs was covalently post-modified with aniline for the selective synthesis of a diazo compound in the presence of nitrite ions. During this process, the color of the gel under visible light changed from white to yellow and, thus, the colorimetric mode of the sensor was turned 'ON'. In parallel, the luminescence of the gel under UV light was quenched, which was denoted as the 'OFF' mode of the sensor. This parallel and unambiguous 'ON'/'OFF' sensing of a toxic chemical (nitrite ions, with a detection limit of 3 μM) was also achieved even in presence of other relevant interfering ions and at concentrations well below the permissible limit (65 μM) set by the World Health Organization (WHO). Furthermore, this chemically reactive luminescent gel could be of potential interest in a wide range of basic and applied contexts.

Gold nanoclusters/graphene quantum dots complex-based dual-emitting ratiometric fluorescence probe for the determination of glucose

Herein, we report the design of a single-excitation/double-emission ratiometric fluorescence nanosensor for the determination of glucose. The sensing system combines glucose oxidation catalyzed by glucose oxidase, Fenton chemistry, Fe³⁺-sensitive fluorescent gold nanoclusters (AuNCs), and Fe³⁺-inert fluorescent graphene quantum dots (GQDs). We used orange-fluorescent AuNCs co-modified with bovine serum albumin and 3-mercaptopropionic acid as the indicator probe, and GQDs with the same excitation wavelength as the BSA/MPA-AuNCs, but with different emission wavelength, as the reference probe. The fluorescence intensity-ratio between 420 nm and 575 nm (F₄₂₀/F₅₇₅) was used to quantitatively determine glucose with a low detection limit of 0.18 μM, and the nanosensor was successfully used to detect glucose in human serum. This ratiometric fluorescence sensing system, based on AuNCs and GQDs, ensures sensitive and convenient determination of glucose, and has broad application prospects for biomedical-analysis applications.

Intrinsically ESIPT-exhibiting and enhanced emission in polymer nanoparticles as signaling for sensing nitrite

A straightforward approach to the fabrication of intrinsically excited-state intramolecular proton transfer (ESIPT)-fluorescent polymer nanoparticles (e-PNPs) was developed. The e-PNPs were obtained by self-assembly of the homopolymers derived from 4-aminosalicylic acid in aqueous solution. By incorporating ESIPT modules into polymer nanoparticles, the ESIPT reaction can be endowed with moderate hydrophobic micro-environment by nanoparticle scaffolds, eliciting enhanced ESIPT emission. The newly developed e-PNPs exhibit strong tautomeric fluorescence(e-FL), good photostability, low-toxicity and favourable biocompatibility in aqueous solution. Upon the addition of NO₂^-, the e-FL can be significantly quenched owing to the reaction of NO₂^- with the amide groups on e-PNPs. From this basis, the fluorescence detection of NO₂^- was implemented, which showed a linear relationship between 0 nM and 110 nM with a detection limit of 2.3 nM. Furthermore, e-PNPs were used as nanoprobes to monitor the NO₂^- levels in HeLa cells by fluorescence imaging, demonstrating the ability of discrimination from different concentrations of NO₂^-. The proposed method can be applied to a wide range of other ESIPT modules to integrate into polymer nanoparticles and offer highly sensitive nanosensing platform for bioanalysis and molecular imaging.

Blue emitting nitrogen-doped carbon dots as a fluorescent probe for nitrite ion sensing and cell-imaging

This study examined the efficiency of pH-dependent, fluorescent carbon dots for the sensing of hazardous anions in aqueous media and cell imaging. The nitrite anion, an important water-soluble element for environmental and biological systems, requires continuous monitoring because a high concentration can affect the systems severely. The as-synthesized carbon dots efficiently detected the nitrite anion in aqueous solution through a fluorescent 'Turn Off' phenomenon. The quenching mechanism was investigated through proper microscopic and spectroscopic studies. The limit of detection and linear detection range were 7.9 nM and 2.3μM-7.7 mM, respectively. The sensitivity was tested with different water samples. In a parallel experiment, the as-synthesized carbon dots were used as a cell-imaging probe for HeLa cells, highlighting their potential in different biological studies.

Tailoring the photoluminescence of atomically precise nanoclusters

Due to their atomically precise structures and intriguing chemical/physical properties, metal nanoclusters are an emerging class of modular nanomaterials. Photo-luminescence (PL) is one of their most fascinating properties, due to the plethora of promising PL-based applications, such as chemical sensing, bio-imaging, cell labeling, phototherapy, drug delivery, and so on. However, the PL of most current nanoclusters is still unsatisfactory-the PL quantum yield (QY) is relatively low (generally lower than 20%), the emission lifetimes are generally in the nanosecond range, and the emitted color is always red (emission wavelengths of above 630 nm). To address these shortcomings, several strategies have been adopted, and are reviewed herein: capped-ligand engineering, metallic kernel alloying, aggregation-induced emission, self-assembly of nanocluster building blocks into cluster-based networks, and adjustments on external environment factors. We further review promising applications of these fluorescent nanoclusters, with particular focus on their potential to impact the fields of chemical sensing, bio-imaging, and bio-labeling. Finally, scope for improvements and future perspectives of these novel nanomaterials are highlighted as well. Our intended audience is the broader scientific community interested in the fluorescence of metal nanoclusters, and our review hopefully opens up new horizons for these scientists to manipulate PL properties of nanoclusters. This review is based on publications available up to December 2018.

Enzyme-Based Ultrasensitive Electrochemical Biosensors for Rapid Assessment of Nitrite Toxicity: Recent Advances and Perspectives

In the present era of rapid international globalization and industrialization, intensive use of nitrite as a fertilizing agent in agriculture, preservative, dyeing agent, food additive and as corrosion inhibitor in industrial sectors is adversely effecting environment, natural habitats and human health. The issue of toxicity and carcinogenicity due to excessive ingestion of nitrites via the dietary intake has led to an imminent need for its efficient real-time monitoring in situ. Nitrite detection employing electrochemical biosensors has been gaining high credibility in the field of clinical research. Nitrite biosensors have emerged as an outstanding choice for portable point of care testing of nitrite quantification owing to the excellent properties, such as rapidity, miniaturization, ultra-low limits of detection, multiplexing and enhanced detection sensitivity. The article is enclosed with an interesting outlook on latest emerging trends in the development of nitrite biosensors utilizing nanomaterials, such as metal nanoparticles, carbon nanotubes, metal oxide nanoparticles, nanocomposites, polymers and biomaterials. The present review embarks on the highlights relevant to the nitrite quantification in real samples, then proceeds with a meticulous description of the most pertinent electrochemical nitrite biosensors, which have been proposed by adopting diverse materials and strategies of fabrication and finally end with the achievements and future outlook signifying the application of these nanoengineered biosensors for environmental surveillance and human safety.

A biomimetic approach to conjugate vitamin B6 cofactor with the lysozyme cocooned fluorescent AuNCs and its application in turn-on sensing of zinc(II) in environmental and biological samples

This communication focusses on the synthesis of red fluorescent lysozyme cocooned gold nanoclusters (Lyso-AuNCs) that have been successfully applied for the selective and specific recognition of the vitamin B₆ cofactor pyridoxal-5'-phosphate (PLP). The red fluorescence of Lyso-AuNCs showed remarkable color change to yellow upon conjugation with PLP due to the formation of a Schiff base between the free -NH₂ present in the lysozyme and the -CHO group of PLP. The developed PLP conjugated Lyso-AuNCs (PLP_Lyso-AuNCs) was applied for the selective turn-on recognition of Zn²⁺ ions in aqueous medium. The yellow fluorescence of PLP_Lyso-AuNCs exhibited significant enhancement at 475 nm in the presence of Zn²⁺ producing bluish-green fluorescence attributed to the complexation-induced aggregation of nanoclusters. The nanoprobe exhibits nanomolar limit of detection for Zn²⁺ ions (39.2 nM) and the practicality of the nanoprobe was validated in various environmental water samples and biological plasma, urine, and beetroot extract, with fairly good recovery percent. Also, the system was successfully implemented for the intracellular detection and monitoring of Zn²⁺ in live HeLa cells. Graphical abstract Applications of red emitting lysozyme cocooned gold nanoclusters (Lyso-AuNCs) for the selective recognition of the vitamin B₆ cofactor pyridoxal-5'-phosphate (PLP) and the conjugated nano-assembly PLP_Lyso-AuNCs for turn-on detection of Zn²⁺ ions in various environmental and biological samples.

A simple fluorescence sensor for the detection of nitrite (NO2−) in real samples using water-dispersible graphite-like carbon nitride (w-g-C3N4) nanomaterials

A novel NO₂⁻ detection method based on the reaction of w-g-C₃N₄ with nitrite (NO₂⁻) in an acidic medium.

Fluorescent metal quantum clusters: an updated overview of the synthesis, properties, and biological applications

A brief history of metal quantum clusters, their synthesis methods, physical properties, and an updated overview of their applications is provided.

Metal nanoclusters: novel probes for diagnostic and therapeutic applications

Metal nanoclusters, composed of several to a few hundred metal atoms, have received worldwide attention due to their extraordinary physical and chemical characteristics. Recently, great efforts have been devoted to the exploration of the potential diagnostic and therapeutic applications of metal nanoclusters. Here we focus on the recent advances and new horizons in this area, and introduce the rising progress on the use of metal nanoclusters for biological analysis, biological imaging, therapeutic applications, DNA assembly and logic gate construction, enzyme mimic catalysis, as well as thermometers and pH meters. Furthermore, the future challenges in the construction of biofunctional metal nanoclusters for diagnostic and therapeutic applications are also discussed. We expect that the rapidly growing interest in metal nanocluster-based theranostic applications will certainly not only fuel the excitement and stimulate research in this highly active field, but also inspire broader concerns across various disciplines.

Choline-induced selective fluorescence quenching of acetylcholinesterase conjugated Au@BSA clusters

We have developed a highly selective sensitive fluorescent detection of acetylcholine (ACh) using bovine serum albumin (BSA) protected atomically precise clusters of gold. The gold quantum clusters (AuQC@BSA) synthesized using bovine serum albumin and conjugated with acetylcholinesterase (AChE), an enzyme specific for acetylcholine, resulting in AuQC@BSA-AChE. The enzyme, AChE hydrolyzes acetylcholine (ACh) to choline (Ch) which in turn interacts with AuQC@BSA-AChE and quenches its fluorescence, enabling sensing. We have carried out the real time monitoring of the hydrolysis of ACh using electrospray ionization mass spectrometry (ESI MS) to find out the mechanism of fluorescent quenching. The validity of present method for determination of concentration of acetylcholine in real system such as blood was demonstrated. Further, the sensor, AuQC@BSA-AChE can be easily coated on paper and an efficient and cheap sensor can be developed and detection limit for ACh is found to be 10nM. The fluorescent intensity of AuQC@BSA-AChE is sensitive towards acetylcholine in range of 10nM to 6.4µM. This suggests that AuQC@BSA-AChE has an excellent potential to be used for diagnosis of various neuropsychological and neuropsychiatric disorders.

Metal nanoclusters: applications in environmental monitoring and cancer therapy

Metal nanoclusters (NCs), with dimensions between metal atoms and nanoparticles, have attracted more and more attention due to their unique physical and chemical properties. With their size approaching the Fermi wavelength of electrons, metal NCs possess molecule-like properties and excellent fluorescence emission. Owing to their ultrasmall size, strong fluorescence, and excellent biocompatibility, they have been widely studied in environmental and biological fields concerning their applications. In this review, we will introduce the properties of metal NCs, mainly focusing on the synthesis of metal alloy NCs and the recent progress in their applications in environmental monitoring and cancer therapy.

Hierarchical logic structures based on responsive fluorescent gold nanoclusters

Logic with Gold Clusters. Gold nanoclusters can be used for the development of hierarchical logic structures. This is based on the reversible change in the fluorescence of the clusters at different pH of the medium, its temperature and metal ion concentration dependence.

A label-free fluorescent assay for free chlorine in drinking water based on protein-stabilized gold nanoclusters

Bovine serum albumin stabilized Au nanoclusters (BSA-AuNCs) were demonstrated as a novel fluorescence probe for sensitive and selective detection of free chlorine in drinking water. The fluorescence of BSA-AuNCs was found to be quenched effectively by the free chlorine, and the decrease in fluorescence intensity of BSA-AuNCs allowed the sensitive detection of free chlorine in the range of 0.8-800 μM. The detection limit is 0.50 μM at a signal-to-noise ratio of 3. The present fluorescent assay for free chlorine possesses low detection limit, wide linear range and good selectivity. Real tap water samples were analyzed with satisfactory results, which suggested its potential for water quality analysis.

Self-assembly of hybridized ligands on gold nanodots: tunable photoluminescence and sensing of nitrite

Highly photoluminescent gold nanodots (Au NDs) via etching and co-deposition of hybridized ligands [11-mercaptoundecanol (11-MU) and its complexes with amphiphilic ligands] on gold nanoparticles (∼3 nm) have been prepared and employed for the detection of nitrite based on the analyte-induced photoluminescence quenching.

Optical reading of contaminants in aqueous media based on gold nanoparticles

With increasing trends of global population growth, urbanization, pollution over-exploitation, and climate change, the safe water supply has become a global issue and is threatening our society in terms of sustainable development. Therefore, there is a growing need for a water-monitoring platform with the capability of rapidness, specificity, low-cost, and robustness. This review summarizes the recent developments in the design and application of gold nanoparticles (AuNPs) based optical assays to detect contaminants in aqueous media with a high performance. First, a brief discussion on the correlation between the optical reading strategy and the optical properties of AuNPs is presented. Then, we summarize the principle behind AuNP-based optical assays to detect different contaminants, such as toxic metal ion, anion, and pesticides, according to different optical reading strategies: colorimetry, scattering, and fluorescence. Finally, the comparison of these assays and the outlook of AuNP-based optical detection are discussed.

A universal lateral flow biosensor for proteins and DNAs based on the conformational change of hairpin oligonucleotide and its use for logic gate operations

A universal lateral flow biosensor for proteins and DNAs was designed on the base of target-induced conformational changes of hairpin oligonucleotide (HO). CEA (carcinoembryonic antigen) protein and c-DNA were detected both with the naked eye and a strip reader. The scheme of detecting proteins and DNAs were based on the unique molecular recognition properties of HO to the targets to form different quantities of "active" biotin groups on the surface of gold nanoparticles (AuNPs). The output of the strip is the color of the test line, which inspired us to combine strip biosensor with logic gate. Two strip logic gates ("OR" and "INH") were designed in our paper and the combinatorial logic gates in our paper could be used to make high-throughput judgment about what targets were present in the input samples according to the output results. The biosensor facilitates a portable analysis at ambient temperature as it is simple to be conducted and no requirement of training is needed. The strip logic system is proved an excellent selection and can operate effectively as well as in human serum samples. Therefore, we indicate that such logic strips a foreseeable promise in application of intelligent point-of-care and in-field diagnostics.