Construction of CDs@ß-CD@CCM ratiometric fluorescence probe for FRET-based ClO--sensing.

ß-Cyclodextrin (ß-CD)-functionalized carbon quantum dots (CDs) loaded with curcumin (CCM) were used for ClO-sensing with high sensitivity and selectivity. This fluorescence resonance energy transfer (FRET)-based sensor was created through attaching CCM to the CDs via ß-CD linker. CCM could get into the interior of ß-CD triggering the FRET from CDs to CCM, providing an 'off' state of the CDs. However, the effect of FRET was weakened by the ClO-, because the o-methoxyphenol structure from CCM was oxidized to be benzoquinone. The fluorescence intensity of CDs@ß-CD@CCM at 440 nm can be heightened and 520 nm from CCM can decrease along with the increased ClO-. Therefore, a ratiometric fluorescence probe for ClO-sensing is successfully constructed. It conforms to a polynomial curve equation which is I440/I520= -0.0268 + 0.0315 CClO-+ 0.0055[CClO-]2(R2= 0.9958) between 0 and 18.4µM ClO-. Furthermore, we also obtain excellent results using this spectrophotometric method for ClO--sensing in pure water and commercial disinfectants, which afford potential in the environment monitoring area. We expect this sensing platform could be helpful in other analogous probes in relevant fields.

Conjugated Polymers Based on Carbazole and Tridentate Ligands as the "On-Off-On" Fluorescent Probes for Detection of Ni (II) Ion and Lysine.

Two novel conjugated polymers (polymer 1 and polymer 2) containing trisheterocyclic systems and carbazole as the copolymerization unit were synthesized by the Suzuki coupling reaction and characterized using NMR spectroscopy and other methods. 4'-(3,5-Dibromophenyl)-2,2':6',2''-terpyridine and 2,2'-(4-(3,5-dibromophenyl)pyridine-2,6-diyl)dithiazole were used as the recognizing units of the two polymers respectively. The polymers show blue-violet fluorescence when dissolved in THF. The ability of the polymers to identify anions and metal ions was investigated by fluorescence sensing tests. It was found that I- not only quenched the fluorescence but also undergone some redshift. Ni (II) efficiently quenched the fluorescence of the polymers, and polymer 2 recognized Ni2+ with higher specificity. UV-visible absorption titration experiments showed that Ni2+ formed complexes with the polymers. In addition, the formation of complexes between Ni2+ and polymers were used for the detection of amino acids, and it was found that lysine could regenerate the fluorescence of [polymer 1-Ni2+] and [polymer 2-Ni2+] with 99% fluorescence recovery.

Analytical control of imatinib in bioanalytical samples using graphene quantum dots sensing.

An analytical method for the determination of imatinib (IMA, the primary treatment for chronic myeloid leukemia), based on the fluorescence properties of graphene quantum dots (GQDs), is reported in this work. The method is addressed to the analytical control of IMA in biological and pharmaceutical samples, due to the present interest in the control of the doses of this anticancer drug, as well as the therapeutic monitoring. The whole method involves the use of a solid-phase extraction (SPE) procedure, followed by an evaporation step, for the treatment of biological samples. For that, tC18 sorbent cartridges were used. After the sample treatment, the solution containing the analyte was mixed with an aqueous solution of GQDs at pH 7.2, and the fluorescent quenching of GQDs was measured. IMA was determined in the 10-250 µg L-1 range, with a limit of detection of 21 µg L-1 and a precision of 1.5% as relative standard deviation, measured in terms of reproducibility. The recovery for biological samples was in the 84-113% range.

A smartphone sensing platform for the sensitive and selective detection of clothianidin based on MIP-functionalized lanthanide MOF.

A novel molecularly imprinted nanomaterial (Eu (BTC)-MPS@MIP) was synthesized on the surface of silanized europium-based metal-organic frameworks (Eu (BTC)-MPS) using 1, 3, 5-benzotrioic acid (H3BTC) as a ligand. The resulting Eu (BTC)-MPS@MIP was applied to constructing a smartphone sensing platform for the sensitive and selective detection of clothianidin (CLT) in vegetables. The synthesized Eu (BTC)-MPS@MIP demonstrated the successful formation of a typical core-shell structure featuring a shell thickness of approximately 70 - 80 nm. The developed sensing platform based on Eu (BTC)-MPS@MIP exhibited sensitivity in CLT detection with a detection limit of 4 µg/L and a linear response in the range 0.01 - 10 mg/L at excitation and emission wavelengths of 365 nm and 617 nm, respectively. The fluorescence sensing platform displayed excellent specificity for CLT detection, as evidenced by a high imprinting factor of 3.1. This specificity is primarily attributed to the recognition sites in the molecularly imprinted polymer (MIP) layer. When applied to spiked vegetable samples, the recovery of CLT ranged from 78.9 to 102.0%, with relative standard deviation (RSD) values falling between 2.2 and 6.2%. The quenching mechanism of Eu (BTC)-MPS@MIP toward CLT can be attributed to the inner filter effect (IFE), resulting from the optimal spectral overlap between the absorption spectrum of CLT and the excitation spectra of Eu (BTC)-MPS@MIP. The proposed method has the potential for extension to the detection of other pesticides by replacing the MIP recognition probes.

Highly Sensitive Fluorescent Sensing for Nitrobenzene of CdII Complexes Based on Three Isomers and a Bis-Imidazole Ligand.

Detection of nitro pollutants is an important topic in environmental protection. A total of 3 Cd (II) complexes (1-3) based on 3 soft organic isomers, n-(3,5-dicarboxylato benzyloxy) benzoic acid (n = 2, 3 or 4-H3DBB), and a linear N-donor ligand, 3-bis(imidazole-l-ylmethyl) benzene (3-bibz), have been synthesized hydrothermally. Structural diversity of Complexes 1-3 displays the architectural 2D or 3D change: Complex 1 exhibits a 2D network featuring tri-nuclear metal units, Complex 2 is a 3D framework based on similar tri-nuclear metal units, and Complex 3 shows a 3D network with binuclear units. Fluorescent sensing properties exhibited in all these complexes have been discovered to detect nitrobenzene (NB) selectively and sensitively. In particular, Complex 3 possesses high sensitivity for NB with the lowest detection limit of 1.15 × 10-10 M. The results of the theoretical calculation verified the fluorescence detection mechanism of NB by these Cd-based complexes. Therefore, these Cd-based complexes might be used as excellent luminescent sensors for NB.

Chemical Approaches to Optimize the Properties of Organic Fluorophores for Imaging and Sensing.

Organic fluorophores are indispensable tools in cells, tissue and in vivo imaging, and have enabled much progress in the wide range of biological and biomedical fields. However, many available dyes suffer from insufficient performances, such as short absorption and emission wavelength, low brightness, poor stability, small Stokes shift, and unsuitable permeability, restricting their application in advanced imaging technology and complex imaging. Over the past two decades, many efforts have been made to improve these performances of fluorophores. Starting with the luminescence principle of fluorophores, this review clarifies the mechanisms of the insufficient performance for traditional fluorophores to a certain extent, systematically summarizes the modified approaches of optimizing properties, highlights the typical applications of the improved fluorophores in imaging and sensing, and indicates existing problems and challenges in this area. This progress not only proves the significance of improving fluorophores properties, but also provide a theoretical guidance for the development of high-performance fluorophores.

Stimulus-Responsive Lanthanide MOF Materials Encapsulated with Viologen Derivatives: Characterization, Photophysical Properties and Sensing on Nitrophenols.

Viologens (1,1'-disubstituted 4,4'-bipyridyls) possessing electron-deficient properties and redox activity are a class of suitable chromophores to assemble metal-organic hybrid photochromic materials. Thus, viologen-functionalized metal-organic frameworks (MOFs) have attracted much attention for their photochromic properties; however, the syntheses of lanthanide-viologen hybrid crystalline photochromic materials still face many challenges. For example, the structures and properties of the final products are difficult to predict and are limited by molecular configurations. In this work, host-guest composite-material Ln-NH2 BDC-pbpy MOFs were constructed by encapsulating viologen derivative pbpyCl2 . The pbpy2+ moieties are uniformly embed by their π-π conjugation in the pores of the 3D structure by electrostatic interactions. Due to the encapsulation of the chromophore pbpy2+ moieties, Ln-NH2 BDC-pbpy MOFs have reversible photochromic properties: they can change color after irradiation and can return to the original color after being protected from light or heating. Interestingly, the fluorescence intensity decreases with illumination time and recovers in the dark. As a result, Ln-NH2 BDC-pbpy MOFs show both photochromic and photomodulated fluorescence. Based on the outstanding fluorescence performance of the Ln-NH2 BDC-pbpy MOFs, they also show a wonderful effect for detecting nitrophenols, especially TNP.

Fast, Selective and Sensitive Fluorescence Detection of Levofloxacin, Fe3+ and Cu2+ Ions in 100% Aqueous Solution Via Their Reciprocal Recognition.

The fluorescence detection of ions and pharmaceutical effluents by using organic chemosensors is a valuable surrogate to the currently existing expensive analytical methods. In this regard, the design of multi-functional chemosensors to recognize desirable guests is of utmost importance. In this study, we first show that levofloxacin (LVO) is able to use as a fluorescent chemosensor for the detection of biologically important Cu2+ (turn-off) and Fe3+ (turn-on) ions via independent signal outputs in 100% aqueous buffer solutions. Next, using the reciprocal recognition of LVO and Fe3+ provides a unique emission pattern for the detection of LVO. This approach exhibited a high specificity to LVO among various pharmaceutical samples, namely acetaminophen (AC), azithromycin (AZ), gemifloxacin (GEM) and ciprofloxacin (CIP) and also showed great anti-interference property in urine. The attractive features of this sensing system are availability, easy-to-use, high sensitivity (limit of detection = 18 nM for Cu2+, 22 nM for Fe3+ and 0.12 nM for LVO), rapid response (5 s) with an excellent selectivity. Levofloxacin (LVO) is able to use as a fluorescent chemosensor for the detection of Cu2+ (turn-off) and Fe3+ (turn-on) ions via independent signal outputs. Moreover, using the reciprocal recognition of LVO and Fe3+ a unique emission pattern for the detection of LVO was achieved which is applicable for biological samples. The attractive features of this sensing system are availability, easy-to-use, high sensitivity, rapid response (5 s) with an excellent selectivity.

Dual mode detection and sunlight-driven photocatalytic degradation of tetracycline with tailor-made N-doped carbon dots.

Carbon dots have emerged as one of the most promising materials with various potential applications derived from their unique photophysical and chemical properties. The present work investigates the electrochemical and photochemical properties of one-pot synthesized carbon dots for environmental sustainability. Facile microwave-assisted pyrolysis of urea and glucose yielded nitrogen doped carbon dots (N-doped carbon dots) with blue fluorescence and a quantum yield of 14.9%. As synthesized N- doped carbon dot had intense fluorescence, stability, water solubility, and biocompatibility. In the sensing studies, N-doped carbon dots appeared as a dual sensor for drug tetracycline with excellent sensitivity and selectivity. Beyond sense, the carbon dots have the potential to act as a photocatalyst for the degradation of tetracycline. Further, N-doped carbon dot could bring exhaustive degradation of tetracycline (>95%) within 10 min in the absence of any additives. This is the first time report on the utilization of raw non-metal doped carbon dots as a photocatalyst for the degradation of tetracycline.

Visible Light and Microwave-Mediated Rapid Trapping and Release of Singlet Oxygen Using a Coordination Polymer.

Due to its high reactivity and oxidative strength, singlet oxygen (1 O2 ) is used in a variety of fields including organic synthesis, biomedicine, photodynamic therapy and materials science. Despite its importance, the controlled trapping and release of 1 O2 is extremely challenging. Herein, we describe a one-dimensional coordination polymer, CP1, which upon irradiation with visible light, transforms 3 O2 (triplet oxygen) to 1 O2 . CP1 consists of CdII centers bridged by 9,10-bis((E)-2-(pyridin-4-yl)vinyl)anthracene ligands which undergo a [4+2] cycloaddition reaction with 1 O2 , resulting in the generation of CP1-1 O2 . Using microwave irradiation, CP1-1 O2 displays efficient release of 1 O2 , over a period of 30 s. In addition, CP1 exhibits enhanced fluorescence and has an oxygen detection limit of 97.4 ppm. Theoretical calculations reveal that the fluorescence behaviour is dominated by unique through-space conjugation. In addition to describing a highly efficient approach for the trapping and controlled release of 1 O2 , using coordination polymers, this work also provides encouragement for the development of efficient fluorescent oxygen sensors.

Metal ion-mediated carbon dot nanoprobe for fluorescent turn-on sensing of N-acetyl-l-cysteine.

In this work, carbon dots (CDs) was easily synthesized from aspartic acid through a pyrolysis method. Based on their favourable fluorescence properties, CDs were utilized to design a metal ion-mediated fluorescent probe for N-acetyl-l-cysteine (NAC) detection. The fluorescence intensity of CDs was firstly quenched by manganese ions (Mn2+ ) through static quenching effect and subsequently restored by NAC via the combination with Mn2+ due to the coordination effect. Therefore, the fluorescent turn-on sensing of NAC was actuated based on the fluorescence quenching stimulated by Mn2+ and recovery induced by coordination. The fluorescence recovery efficiencies showed a proportional range to the concentration of NAC in the range 0.04-5 mmol L-1 and the detection limit was 0.03 mmol L-1 . Furthermore, this metal ion-mediated fluorescent nanoprobe was applied to human urine sample detection and the standard recovery rates were located in the range 97.62-102.34%. This was the first time that Mn2+ was used to construct a fluorescent nanoprobe for NAC. Compared with other heavy metal ions, Mn2+ with good biosecurity prevented the risk of application, which made the nanoprobe green and biopractical. The facile synthesis of CDs and novel metal ion-mediated sensing mode made it a promising method for pharmaceutical analysis.

Preparation of Nitrogen and Sulfur Co-Doped Fluorescent Carbon Dots from Cellulose Nanocrystals as a Sensor for the Detection of Rutin.

The poor water solubility, large particle size, and low accessibility of cellulose, the most abundant bioresource, have restricted its generalization to carbon dots (CDs). Herein, nitrogen and sulfur co-doped fluorescent carbon dots (N, S-CDs) were hydrothermally synthesized using cellulose nanocrystals (CNC) as a carbon precursor, exhibiting a small particle size and excellent aqueous dispersion. Thiourea was selected as a nitrogen and sulfur dopant to introduce abundant fluorescent functional groups into N, S-CDs. The resulting N, S-CDs exhibited nanoscale size (6.2 nm), abundant functional groups, bright blue fluorescence, high quantum yield (QY = 27.4%), and high overall yield (16.2%). The excellent optical properties of N, S-CDs endowed it to potentially display a highly sensitive fluorescence "turn off" response to rutin. The fluorescence response for rutin allowed a wide linear range of 0-40 mg·L-1, with a limit of detection (LOD) of 0.02 µM, which revealed the potential of N, S-CDs as a rapid and simple sensing platform for rutin detection. In addition, the sustainable and large-scale production of the N, S-CDs in this study paves the way for the successful high-value utilization of cellulose.

A Dual-emitting Two-dimensional Nickel-based Metal-organic Framework Nanosheets: Eu3+/Ag+ Functionalization Synthesis and Ratiometric Sensing in Aqueous Solution.

Using two-dimensional (2D) nickel-based metal organic framework (Ni-MOF) nanosheets as a matrix, Eu3+ and Ag+ were incorporated to synthesize Ag/Eu@Ni-MOF with double luminescence centers of Eu3+ ion (615 nm) and organic ligand (524 nm). And a ratiometric luminescence sensor is constructed based on Ag/Eu@Ni-MOF for sensitive detection of biothiols in aqueous solutions. The dual-emissive fluorescence properties can be tuned by changing the amounts of Ag+ ions doping. The results of temperature and pH effects on the fluorescence of Ag/Eu@Ni-MOF indicates that the Ag/Eu@Ni-MOF is a temperature-sensitive material and the fluorescence of Ag/Eu@Ni-MOF can keep stable over a wide pH range. Due to the binding of -SH in cysteine (Cys) and glutathione (GSH) with Ag+, the ligand luminescence was significantly inhibited by weakening the Ag + influence on the energy transfer process in the MOFs. Therefore, ratiometric fluorescent sensing of biomolecular thiols was realized based on the dual-emission Ag/Eu@Ni-MOF. More importantly, the fluorescence color change can be observed with naked eyes to realize visual detection. The ratiometric fluorescent sensor exhibits high performance for Cys and GSH detection with a wide linear range of 5-250 µM and a relatively low detection limit of 0.20 µM and 0.17 µM, respectively. Furthermore, the biothiols content in human serum was determined with satisfactory results. It proves the Ni-MOF nanosheets can be used as a stable matrix for construction luminescent MOFs for the first time, and validate the great potential of Ag/Eu@Ni-MOF as a ratiometric fluorescent probe for point-of-care testing (POCT) in disease diagnosis.

Molecularly Imprinted Micelles for Fluorescent Sensing of Nonsteroidal Anti-Inflammatory Drugs (NSAIDs).

Nonsteroidal anti-inflammatory drugs (NSAIDs) are widely used over-the-counter drugs and their uncontrolled disposal is a significant environmental concern. Although their fluorescent sensing is a desirable method of detection for its sensitivity and simplicity, the structural similarity of the drugs makes the design of selective sensors highly challenging. A thiourea-based fluorescent functional monomer was identified in this work to enable highly efficient synthesis of molecularly imprinted nanoparticle (MINP) sensors for NSAIDs such as Indomethacin or Tolmetin. Micromolar binding affinities were obtained in aqueous solution, with binding selectivities comparable to those reported for polyclonal antibodies. The detection limit was ~50 ng/mL in aqueous solution, and common carboxylic acids such as acetic acid, benzoic acid, and citric acid showed negligible interference.

Recent progress on graphene quantum dots-based fluorescence sensors for food safety and quality assessment applications.

The versatile photophysicalproperties, high surface-to-volume ratio, superior photostability, higher biocompatibility, and availability of active sites make graphene quantum dots (GQDs) an ideal candidate for applications in sensing, bioimaging, photocatalysis, energy storage, and flexible electronics. GQDs-based sensors involve luminescence sensors, electrochemical sensors, optical biosensors, electrochemical biosensors, and photoelectrochemical biosensors. Although plenty of sensing strategies have been developed using GQDs for biosensing and environmental applications, the use of GQDs-based fluorescence techniques remains unexplored or underutilized in the field of food science and technology. To the best of our knowledge, comprehensive review of the GQDs-based fluorescence sensing applications concerning food quality analysis has not yet been done. This review article focuses on the recent progress on the synthesis strategies, electronic properties, and fluorescence mechanisms of GQDs. The various GQDs-based fluorescence detection strategies involving Förster resonance energy transfer- or inner filter effect-driven fluorescence turn-on and turn-off response mechanisms toward trace-level detection of toxic metal ions, toxic adulterants, and banned chemical substances in foodstuffs are summarized. The challenges associated with the pretreatment steps of complex food matrices and prospects and challenges associated with the GQDs-based fluorescent probes are discussed. This review could serve as a precedent for further advancement in interdisciplinary research involving the development of versatile GQDs-based fluorescent probes toward food science and technology applications.

Hydrosulfide (HS- ) Recognition and Sensing in Water by Halogen Bonding Hosts.

Hydrogen sulfide (H2 S) plays a crucial signalling role in a variety of physiological systems, existing as the hydrosulfide anion (HS- ) at physiological pH. Combining the potency of halogen bonding (XB) for anion recognition in water with coumarin fluorophore incorporation in acyclic host structural design, the first XB receptors to bind and, more importantly, sense the hydrosulfide anion in pure water in a reversible chemosensing fashion are demonstrated. The XB receptors exhibit characteristic selective quenching of fluorescence upon binding to HS- . Computational DFT and molecular dynamics simulations in water corroborate the experimental anion binding observations, revealing the mode and nature of HS- recognition by the XB receptors.

Sulfonation of 3,3'-Diformyl-BINOL for Enantioselective Fluorescent Recognition of Amino Acids in Water.

Reaction of (R)-3,3'-diformyl-1,1'-bi-2-naphthol with concentrated sulfuric acid gives the corresponding 6,6'-disulfonated compound (R)-2 selectively. This provides a simple and efficient method to convert a water-insoluble compound to a water-soluble fluorescent probe. It is found that (R)-2 in combination with Zn2+ shows a highly enantioselective fluorescent response toward various amino acids in the aqueous HEPES buffer solution at pH 7.4. For example, an enantioselective fluorescence enhancement ratio [ef=ΔID /ΔIL ] up to 35.8 is observed for the recognition of asparagine. NMR and mass spectroscopic investigations are conducted to explore the reaction of (R)-2 with asparagine.

Nitrogen-doped graphene quantum dot-based sensing platform for metabolite detection.

A novel fluorescent sensing platform based on nitrogen-doped graphene quantum dots (N-GQDs) is presented, which is able to detect various metabolites (cholesterol, glucose, lactate, and xanthine) rapidly, sensitively, and selectively. Hg2+ can attach on the surface of N-GQDs, leading to the quenching of N-GQD fluorescence. In the presence of cysteine (Cys), Hg2+ is released from N-GQDs and associates with Cys. Then, the fluorescence of N-GQDs is recovered. Hydrogen peroxide, resulting from the enzymatic oxidation of metabolites, can convert two molecules of Cys into one molecule of cystine, which cannot bind with Hg2+. So, the fluorescence of N-GQDs quenched again. For cholesterol, glucose, lactate, and xanthine, the limits of detection are 0.035 µmol/L, 0.025 µmol/L, 0.07 µmol/L, and 0.04 µmol/L, respectively, and the linear ranges are 1-12 µmol/L, 0.06-3 µmol/L, 0.2-70 µmol/L, and 0.12-17 µmol/L, respectively. The presented method was applied to quantify metabolites in human blood samples with satisfactory results. Graphical abstract.

Visualization of hydrogen polysulfides in living cells and in vivo via a near-infrared fluorescent probe.

Hydrogen polysulfides (H2Sn, n > 1), as the oxidized forms of H2S, have attracted increasing attention these years due to their involvement in signaling transduction and cytoprotective processes. It is necessary to detect H2Sn in living systems for the study of their functions. In this work, we report a BODIPY-based near-infrared emitting fluorescence probe NIR-PHS1, with "turn-on" response, rapid response rate (within 10 min), outstanding selectivity and excellent sensitivity (detection limit = 12 nM) response towards H2Sn. The probe was successfully applied to the visualizing of endogenous H2Sn in living cells. Moreover, it can be used for near-infrared in vivo imaging of H2Sn in living mice. Therefore, NIR-PHS1 could be a potential imaging tool to study the biological roles of H2Sn in living systems.

Self-assembled protein-enzyme nanoflower-based fluorescent sensing for protein biomarker.

Multi-protein (or enzyme) conjugates play a vital role in biosensing due to the integrated function of each component, such as biological recognition and signal amplification. In this work, a green self-assembled method for the synthesis of multi-functional protein-enzyme nanoflowers has been developed, in which no chemical modification and coupling reaction is needed to fabricate the fluorescent signal probe. The self-assembled protein-enzyme conjugates streptavidin (SA) -ß-galactosidase (ß-Gal)-CaHPO4 nanoflowers load sufficient enzymes without damaging their activity, which meets the requirements of signal tags for biosensing. Through integrated multi-function of biorecognition (SA) and signal amplification (ß-Gal), the SA-ß-Gal-CaHPO4 hybrid nanoflower-based fluorescent sensor exhibited an ultrasensitive detection of protein biomarker alpha-fetoprotein (AFP), with limits of detection at the fM level. The presented self-assembled strategy can be extensively applied to develop on-demand protein-enzyme conjugates according to the specific requirements in a variety of applications including biosensors, bioimaging, and biomedicine. Graphical abstract A self-assembled method has been presented for the facile and green synthesis of SA-ß-Gal-CaHPO4 nanocomplexes with flower-like shape and high activity, and further employed as signal tag for fluorescent sensing of protein biomarker.