A colorimetric and ratiometric fluorescent sensor for sequentially detecting Cu2+ and arginine based on a coumarin–rhodamine B derivative and its application for bioimaging

In this work, a colorimetric and ratiometric fluorescent sensor based on a coumarin–rhodamine B hybrid for the sequential recognition of Cu²⁺ and arginine (Arg) via the FRET mechanism was designed and synthesized. With the addition of Cu²⁺, the solution displayed a colorimetric change from pale yellow to pink which is discernible by the naked eye. Additionally, the fluorescence intensities of the sensor exhibited ratiometric changes for the detection of Cu²⁺ at 490 and 615 nm under a single excitation wavelength of 350 nm, which corresponded to the emissions of coumarin and rhodamine B moieties, respectively. The fluorescence color change could be visualized from blue to pink. The limits of detection were determined to be as low as 0.50 and 0.47 μM for UV-vis and fluorescence measurements, respectively. More importantly, the sensor not only can recognize Cu²⁺ and form a sensor-Cu²⁺ complex but can also sequentially detect Arg with the resulting complex. The detection limits for Arg were as low as 0.60 μM (UV-vis measurement) and 0.33 μM (fluorescence measurement), respectively. A fluorescence imaging experiment in living cells demonstrated that the fabricated sensor could be utilized in ratiometric fluorescence imaging towards intracellular Cu²⁺, which is promising for the detection of low-level Cu²⁺ and Arg with potentially practical significance.

A FRET-based colorimetric and ratiometric coumarin–rhodamine B fluorescent sensor was designed, and its sensing behaviors for sequentially detecting Cu²⁺ and arginine were studied systematically.     

A signal-on fluorescent aptasensor based on gold nanoparticles for kanamycin detection

This study describes the development, verification and practical application of an aptasensor for the fluorometric detection of kanamycin. Using the nucleic acid aptamer with FAM fluorescent group as the conjugate, using gold nanoparticles as the fluorescence dynamic quenching source, a fluorescence sensor was fabricated through the signal-on method for the micro-detection of kanamycin. The nucleic acid chimera is connected to the fluorophore, and the gold nanoparticles are used as the fluorescence dynamic quenching source under actual conditions. The detection limit of kanamycin is 0.1 pM, and the detection range is 0.1 pM to 0.1 μM. This biosensor works satisfactorily in complex samples with no impurities, which gives this method an obvious advantage over other analytical methods. In addition, the mechanism of action between gold nanoparticles/FAM–aptamer/kanamycin is discussed and studied in depth here. It provides a more thorough analysis and more application possibilities for fluorescence-aptamer biosensing.

This study describes the development, verification and practical application of an aptasensor for the fluorometric detection of kanamycin.     

Development of a FRET-based fluorescence aptasensor for the detection of aflatoxin B1 in contaminated food grain samples

The present study aimed to develop an aptamer-based FRET detection strategy for the specific and sensitive detection of AFB1 in contaminated food grains. The study comprises generation of ssDNA aptamers against AFB1 by whole-cell SELEX and their application in a FRET-based platform utilizing graphene oxide (GO) and quantum dots (QDs). The generated aptamers were characterized to determine their specificity and sensitivity using indirect ELISA where AFB1–OVA was used as a coating antigen. Among the aptamers generated, the ATB1 aptamer showed good reactivity and selectivity against AFB1. This aptamer was further characterized to determine its secondary structure and KD value, which was found to be 5.9 kcal mol⁻¹. The characterized aptamers were conjugated onto Cd/Se quantum dots to develop a fluorimetric system for the detection of aflatoxin B1 using a graphene oxide platform. The presence of graphene oxide quenches the fluorescence ability of the quantum dots due to π–π stacking interactions between the aptamer and GO. Upon target addition, the aptamer forms a complex with aflatoxin B1 thereby restoring the fluorescence intensity. The developed assay shows a linear response from 0.002 μg μl⁻¹ to 0.2 μg μl⁻¹ with a detection limit of 0.004 μg μl⁻¹ for the AFB1 standard toxin and showed no cross-reactivity with other closely related mycotoxins. To validate the reliability of the developed method, several field samples spiked with AFB1 were included in this study and the results obtained were cross verified using a standard commercial AFB1 kit. In conclusion, the developed method may find good utility in routine food testing laboratories for risk assessment of AFB1.

The present study aimed to develop an aptamer-based FRET detection strategy for the specific and sensitive detection of AFB1 in contaminated food grains.     

Optimisation of a gold nanoparticle-based aptasensor integrated with image processing for the colorimetric detection of acephate using response surface methodology

Acephate (Ac) is an organophosphate (OP) compound, which is able to inhibit the activity of acetylcholinesterase. Thus, the aim of this study was to optimize the detection of Ac using a thiolated acephate binding aptamer-citrate capped gold nanoparticle (TABA–Cit-AuNP) sensor that also incorporated an image processing technique. The effects of independent variables, such as the incubation period of TABA–Cit-AuNPs (3–24 h) for binding TABA to Cit-AuNPs, the concentration of phosphate buffer saline (PBS) (0.001–0.01 M), the concentration of thiolated acephate binding aptamer (TABA) (50–200 nM), and the concentration of magnesium sulphate (MgSO₄) (1–300 mM) were investigated. A quadratic model was developed using a central composite design (CCD) from response surface methodology (RSM) to predict the sensing response to Ac. The optimum conditions such as the concentration of PBS (0.01 M), the concentration of TABA (200 nM), the incubation period of TABA–Cit-AuNPs (3 h), and the concentration of MgSO₄ (1 mM) were used to produce a TABA–Cit-AuNPs sensor for the detection of Ac. Under optimal conditions, this sensor showed a detection ranging from 0.01 to 2.73 μM and a limit of detection (LOD) of 0.06 μM. Real sample analysis demonstrated this aptasensor as a good analytical method to detect Ac.

We successfully optimized AuNPs, modified DNA aptamer and magnesium sulphate salt to enhance the selectivity and sensitivity for detection of Ac. The accuracy of the detection was also improved by image processing technique.     

Fluorescent sensor for water based on photo-induced electron transfer and Förster resonance energy transfer: anthracene-(aminomethyl)phenylboronic acid ester-BODIPY structure

An anthracene-(aminomethyl)phenylboronic acid ester-BODIPY (DJ-1) was designed and developed as a fluorescent sensor based on photo-induced electron transfer (PET) and Förster resonance energy transfer (FRET) for the detection of a trace amount of water in solvents, where the anthracene skeleton and BODIPY skeleton are the donor fluorophore and the acceptor fluorophore in the FRET process, respectively. It was found that the addition of water to organic solvents containing DJ-1 causes both the suppression of PET in the anthracene-(aminomethyl)phenylboronic acid ester as the PET-type fluorescent sensor skeleton and the energy transfer from the anthracene skeleton to the BODIPY skeleton through a FRET process, thus resulting in the enhancement of the fluorescence band originating from the BODIPY skeleton. This work demonstrates that the PET/FRET-based fluorescent dye composed of the donor fluorophore possessing PET characteristics and the acceptor fluorophore in the FRET process can act as a fluorescent sensor with a large SS for the detection of a trace amount of water in solvents.

An anthracene-(aminomethyl)phenylboronic acid ester-BODIPY (DJ-1) structure was developed as a fluorescent sensor based on photo-induced electron transfer (PET) and Förster resonance energy transfer (FRET) for detection of water in solvents.     

Rhodamine B derivatives-modified upconversion nanoparticles as a fluorescent turn-off–on sensor for the highly sensitive detection of Cu2+ and pyrophosphate

Rhodamine B derivatives (RBP)-modified UCNPs (UCNPs@mSiO₂–RBP) were developed as a fluorescent turn-off–on sensor based on FRET and IFE to detect Cu²⁺ and pyrophosphate (PPi) with a wide linear response range (0–10 μM for Cu²⁺ and 5–35 μM for PPi, much wider than that reported previously) and high sensibility (117 nM for Cu²⁺ and 70 nM for PPi). The MTT experiments and the bioimaging experiments show its promising prospect in tissue imaging.

A new fluorescent turn-off–on sensor was developed based on the the rhodamine B derivatives (RBP) modified UCNPs to detect Cu²⁺ and pyrophosphate (PPi).     

A fluorescence sensing platform of theophylline based on the interaction of RNA aptamer with graphene oxide

RNA, with a structure similar to DNA, should exhibit similar behaviors when it interacts with graphene. In this work, we designed a sensing platform of theophylline based on the interaction of an RNA aptamer with graphene oxide (GO) using the fluorescence as a sensing signal. Firstly, quantum dots (QDs) were modified with the selected ssRNA that can be used as an aptamer to recognize the theophylline. The fluorescence of QDs will be quenched in the presence of GO due to the noncovalent assembly between ssRNA aptamer and GO, leading to fluorescence resonance energy transfer (FRET) from QDs to GO, fluorescence “turn-off”. Then, in the presence of theophylline, the ssRNA aptamer recognizes theophylline to form a dsRNA–theophylline complex. The weak affinity between the complex and GO makes QDs move away from the GO surface, leading to the fluorescence recovery of QDs, fluorescence “turn-on”. Because of the high fluorescence quenching efficiency, unique structure of GO and specificity of the RNA aptamer, the proposed sensing platform exhibits high sensitivity and excellent selectivity for the determination of theophylline. The excellent performance of the sensor based on GO provides new opportunities for sensitive and selective detection of biorecognition events.

A fluorescent sensing platform of theophylline based on the interaction of an RNA aptamer with GO and CdTe as the signal.     

Ratiometric fluorescent sensors for nitrite detection in the environment based on carbon dot/Rhodamine B systems

A novel carbon dot/Rhodamine B-based ratiometric fluorescent probe was developed for a highly sensitivity and selective detection of nitrite (NO₂⁻). The probe showed colour changes from blue to orange under ultraviolet light in response to NO₂⁻ with a detection limit as low as 67 nM in the range of 0 to 40 μM. A ratiometric fluorescent test paper was successfully prepared using the probe solution, which demonstrated its feasibility towards a rapid and semi-quantitative detection of NO₂⁻ in real samples.

A visual ratiometric fluorescent sensor based on blue carbon dot/Rhodamine B is used to selectively detect NO₂⁻ in the environment.     

PCN-222@g-C3N4 cathodic materials for “signal-off” photoelectrochemical sensing of kanamycin sulfate

A novel cathodic photoelectrochemical (PEC) sensor was developed for the ultrasensitive detection of kanamycin sulfate (KAM) based on the g-C₃N₄ coupled zirconium-based porphyrinic metal–organic framework (PCN-222). Photocathodes made by double n-type semiconductors, which was attributed to the transfer of electrons and holes from g-C₃N₄ broad band to PCN-222 with narrow band gap. The photocurrent decreased when KAM was added, which was conducive to the construction of the PEC sensor. Then, the PCN-222@g-C₃N₄ was used as a photosensitive platform to construct a label-free strategy and ultrasensitive detection of KAM with wide linear range from 1 to 1000 nM and a low detection limit of 0.127 nM. Moreover, this sensing platform shows good selectivity, favourable reproducibility and brilliant stability. The reported sensors provided great potential for the detection of KAM in actual samples.

The PCN-222@g-C₃N₄ was firstly used as a photoelectrically active material for the detection of kanamycin sulfate; The sensor has lower detection limit and the broad detection range for kanamycin sulfate.     

An aptasensor for the label-free detection of thrombin based on turn-on fluorescent DNA-templated Cu/Ag nanoclusters

A highly sensitive thrombin aptasensor was constructed based on the alteration of the aptamer conformation induced by the target recognition and the turn-on fluorescence due to the proximity of two darkish DNA-templated copper/silver nanoclusters (DNA-Cu/Ag NCs). Two DNA templates were designed as the functional structures consisting of the Cu/Ag NC-nucleation segment located at two termini or one terminus and the aptamer segment in the middle of a DNA template. Two darkish DNA-Cu/Ag NCs came close to each other when the aptamer combined with the target due to the conformational alteration of the aptamer structure, resulting in an increased fluorescence signal readout. Thrombin was sensitively determined as low as 1.6 nM in the range of 1.6–8.0 nM with a high selectivity. Finally, this sensor succeeded in detecting thrombin in a real fetal bovine serum.

A highly sensitive thrombin aptasensor was constructed based on the alteration of the aptamer conformation induced by the target recognition and the turn-on fluorescence due to the proximity of two darkish DNA-templated copper/silver nanoclusters (DNA-Cu/Ag NCs).     

A novel ratiometric AIEE/ESIPT probe for palladium species detection with ultra-sensitivity

Existing fluorescent probes for palladium (Pd) species detection have revealed their vulnerabilities, such as low sensitivity, poor anti-interference ability and long reaction time. In order to develop a faster and more accurate detection method for palladium species at extremely low concentrations, in this study, we designed a novel ratiometric AIEE/ESIPT probe (HPNI-1) based on the Tsuji–Trost reaction for Pd. According to the data obtained, the probe was able to detect Pd species with an ultra-high anti-interference ability (Pd : other metals = 1 : 1000), rapid detection time (within 2 minute) and ratiometric fluorescent signal changes with a 1.34 nM detection limit. This study not only proves that existing methods can be improved but also provides future prospects for HPNI-1 as one of the greatest probes for Pd species detection.

Existing fluorescent probes for palladium (Pd) species detection have revealed their vulnerabilities, such as low sensitivity, poor anti-interference ability and long reaction time.     

An ‘‘off–on–off’’ sensor for sequential detection of Cu2+ and hydrogen sulfide based on a naphthalimide–rhodamine B derivative and its application in dual-channel cell imaging

A novel colorimetric and fluorometric sensor with unique dual-channel emission to sequentially detect Cu²⁺ and hydrogen sulfide (H₂S) was synthesized from naphthalimide–rhodamine B through the PET and FRET mechanism. The sensor showed a selective “off–on” fluorescence response with a 120-fold increase toward Cu²⁺, and its limits of detection were 0.26 μM and 0.17 μM for UV-vis and fluorescence measurements, respectively. In addition, 1–Cu²⁺ was an efficient “on–off” sensor to detect H₂S with detection limits of 0.40 μM (UV-vis measurement) and 0.23 μM (fluorescence measurement), respectively. Furthermore, the sensor can also be used for biological imaging of intracellular staining in living cells. Therefore, the sensor should be highly promising for the detection of low level Cu²⁺ and H₂S with great potential in many practical applications.

A novel colorimetric and fluorometric sensor with unique dual-channel emission to sequentially detect Cu²⁺ and hydrogen sulfide (H₂S) was synthesized from naphthalimide–rhodamine B through the PET and FRET mechanism.     

A ratiometric and colorimetric probe for detecting Hg2+ based on naphthalimide–rhodamine and its staining function in cell imaging

In this work, a rhodamine derivative was developed as a colorimetric and ratiometric fluorescent probe for Hg²⁺. It exhibited a highly sensitive fluorescence response toward Hg²⁺. Importantly, studies revealed that the probe could be used for ratiometric detection of Hg²⁺, with a low detection limit of 0.679 μM. The mechanism of Hg²⁺ detection using compound 1 was confirmed by ESI-MS, ¹H NMR, and HPLC. Upon the addition of Hg²⁺, the rhodamine receptor was induced to be in the ring-opening form via an Hg²⁺-promoted hydrolysis of rhodamine hydrazide to rhodamine acid. In addition to Hg²⁺ detection, the naphthalimide–rhodamine compound was proven to be effective in cell imaging.

A new probe based on naphthalimide–rhodamine was applied in recognition of Hg²⁺ by a FRET mechanism.     

A ratiometric fluorescence probe for the selective detection of H2S in serum using a pyrene-DPA–Cd2+ complex

A ratiometric and selective hydrogen sulfide (H₂S) detection probe was proposed based on the pyrene-DPA–Cd²⁺ complex through the metal ion displacement approach (MDA) mechanism. While most MDA-based fluorescence probes with paramagnetic Cu²⁺ have focused on the development of a simple turn-on sensor using the broad spectral range of fluorescence enhancement, this ratiometric probe exhibited unchanged monomer emission as a built-in internal reference with an increase in excimer emission with added H₂S. The demonstrated probe showed a rapid response (within 1 min) and a high sensitivity, with 70 nM as the limit of detection. The selectivity for H₂S over cysteine, homocysteine and glutathione was confirmed, and reliable fluorescence enhancement, which could be monitored by the naked eye, was observed upon irradiation with handheld UV light. In addition, this detection system was successfully applied to detect H₂S in human serum without interference from biological molecules.

The pyrene-DPA–Cd²⁺ complex is demonstrated as a ratiometric fluorescence probe for selective hydrogen sulfide detection in serum based on a metal displacement approach.     

A sensitive and selective BINOL based ratiometric fluorescence sensor for the detection of cyanide ions

A highly selective, novel BINOL based sensor BBCN has been developed for the fluorescent ratiometric detection of cyanide ions (CN⁻). The optical study revealed that BBCN exhibited unique spectral changes only with cyanide ions in the presence of other competing ions. Besides, an apparent fluorescent colour change from green to blue was observed. A clear linear relationship was observed between the fluorescence ratiometric ratio of BBCN and the concentration of CN⁻ with a reasonably low detection limit (LOD) of 189 nM (507 ppb). The optical response was due to the nucleophilic addition of CN⁻ to the dicyanovinyl group of the sensor, which compromises the probe''s intramolecular charge transfer. This mechanism was well confirmed by Job''s plot, ¹H-NMR and ESI-MS studies. BBCN showed immediate spectral response towards (1 second) CN⁻ and detection could be realized in a broad pH window. Furthermore, the practical utility of BBCN was studied by test paper-based analysis and the detection of CN⁻ in various water resources.

A highly selective, novel BINOL based sensor BBCN has been developed for the fluorescent ratiometric detection of cyanide ions (CN⁻).     

Adsorption–desorption nano-aptasensors: fluorescent screening assays for ochratoxin A

In this study, a FRET-based fluorescent aptasensor for the detection of ochratoxin A (OTA) was optimized based on the quenching efficiency of single-walled carbon nanotubes (SWCNTs) and the binding affinity of aptamers. OTA aptamers were conjugated with quantum dots and adsorbed to the surface of both acid-modified and unmodified SWCNTs. The maximum fluorescence quenching efficiency of the SWCNTs were compared. Acid-modified SWCNTs (amSWCNTs) have moderate quenching efficiency, providing an optimal sensitivity for qualitative fluorescence-enhancement biosensor assays. The binding parameters of the QD-modified OTA aptamers (1.12.2 and A08min) on the surface of amSWCNTs were compared. Based on our results, the A08min aptamer is a better candidate for OTA detection. Using the A08min aptamer, the SWCNT method had a limit of detection (LOD) of 40 nM. The amSWCNT method had a significantly lower LOD of 14 nM. Turn-on fluorescent nano-aptasensors are emerging as an effective diagnostic tool for simple detection of mycotoxins. Nanocomplexes designed for the detection of mycotoxins in solution and paper-based tests have proven to be useful.

A fluorescent-enhancement biosensor was developed for the mycotoxin ochratoxin A using aptamer-modified quantum dots noncovalently immobilized on carbon nanotubes.     

A ratiometric fluorescence sensor based on enzymatically activatable micellization of TPE derivatives for quantitative detection of alkaline phosphatase activity in serum

A novel ratiometric fluorescence assay via enzymatically activatable micellization in aqueous solution was devised for quantitative detection of alkaline phosphatase (ALP) activity. We demonstrated that the dephosphorylation of the water-soluble, phosphate-functionalized, fluorophore monomer P-TPE-TG, induced by an enzymatic reaction of ALP, leads to micelle formation in aqueous solution because its water-soluble functionality is reduced. The dephosphorylation-induced micellization of P-TPE-TG exhibited a ratiometric sensing response for various ALP concentrations (10–200 mU mL⁻¹) and provided a suitable sensing platform for naked eye detection with increased fluorescence quantum yield (Φ = 3.2%), even compared to a typical TPE-based sensor (Φ = 1.0%), where ALP can be sensed with a detection limit of 0.034 mU mL⁻¹. In addition, P-TPE-TG displayed excellent sensing performance at concentrations from 0 to 50 mU mL⁻¹ in diluted human serum (10%), which offers the capability to exploit ratiometric responses for bioactive substances under practical conditions.

A novel ratiometric fluorescence assay via enzymatically activatable micellization in aqueous solution was devised for quantitative detection of alkaline phosphatase (ALP) activity.     

A FRET-based fluorescent and colorimetric probe for the specific detection of picric acid

Picric acid (PA) as an environmental pollutant and high explosive, has recently received considerable attention. In this paper, a novel fluorescent and colorimetric chemo-probe (L) for the highly selective and sensitive detection of picric acid has been revealed. The probe was facilely constructed using rhodamine B, ethylenediamine and 4-(9H-carbazol-9-yl)benzoyl chloride. Significant fluorescence changes based on an intramolecular fluorescence resonance energy transfer (FRET) effect followed by a distinct color change from colorless to pink were observed after addition of picric acid to the probe solution. Selectivity measurements revealed that the as-synthesized probe exhibited high selectivity toward PA in the presence or absence of other analytes. The experimental titration results suggested that the as-synthesized probe is an effective tool for detection of PA with a nanomolar scale detection limit (820 nM) and could also serve as a “naked-eye” indicator for PA detection.

A FRET-based fluorescent and colorimetric chemo-sensor has been designed and synthesized for the selective and sensitive detection of picric acid.     

A stimuli responsive triplet–triplet annihilation upconversion system and its application as a ratiometric sensor for Fe3+ ions

A ratiometric fluorescent sensor for the detection of Fe³⁺ ions is achieved based on triplet–triplet annihilation upconversion (TTA-UC) luminescence. A new anthracene derivative (named as DHTPA) is designed and synthesized and reveals similar optical properties to 9,10-diphenylanthracene (DPA) and is used as a stimuli responsive annihilator in a TTA-UC system due to its complexation ability. As a result, the UC emission can be significantly quenched by Fe³⁺ ions, while the phosphorescence (PL) emission of sensitizer palladium(ii) octaetylporphyrin (PdOEP) remains nearly constant, which makes the PL signal an appropriate internal reference for the UC signal. The UC and ratio signals (I_UC/I_PL) both reveal a good linear relationship with Fe³⁺ ion concentration, which for the first time makes the TTA-UC system a perfect ratiometric sensor for Fe³⁺ ion detection. This sensing method will open a novel avenue to achieve ratiometric sensors in chemical and biological fields.

A ratiometric fluorescent sensor for detection of Fe³⁺ is achieved based on a triplet–triplet annihilation upconversion (TTA-UC) system with a responsive annihilator.     

A reduced graphene oxide-β-cyclodextrin nanocomposite-based electrode for electrochemical detection of curcumin

Curcumin is a polyphenolic compound with anti-oxidative and anti-cancer properties that is obtained from turmeric plants. Several studies have demonstrated that cancer cells are not killed unless they are exposed to 5–50 mM of curcumin. Consequently, it is vital to control the concentration of curcumin in cancer therapy. In this study, a sensitive electrochemical sensor was fabricated based on a beta-cyclodextrin–reduced graphene oxide (β-CD–rGO) nanocomposite for measuring curcumin concentration. The effects of experimental factors were investigated and the optimum parametric conditions were determined using the Taguchi optimization method. The β-CD–rGO modified electrode exhibited good electrochemical properties for curcumin detection. The results of differential pulse voltammetry experiments unveiled that the sensor shows a linear response to curcumin concentration over the range of 0.05–10 mM with a detection limit of 33 nM and sensitivity of 4.813 μA μM⁻¹. The fabricated sensor exhibited selectivity in the presence of other electroactive species, e.g., propranolol, clomipramine and clonazepam.

In this study, a sensitive electrochemical sensor was fabricated based on a beta-cyclodextrin–reduced graphene oxide (β-CD–rGO) nanocomposite for measuring curcumin concentration.