Jitnapa Sirirak

Highly Hg2+-sensitive and selective fluorescent sensors in aqueous solution and sensors-encapsulated polymeric membrane

Two Hg2+ chemosensors, rhodamine B hydrazide (RBH) and rhodamine 6G hydrazide (R6GH), were synthesized by a single step. In the presence of Hg2+, both of the sensors RBH and R6GH exhibited highly sensitive OFF–ON fluorescence enhancement. Importantly, the sensors showed a selective binding to Hg2+ over other common metal ions such as K+, Na+, Fe3+, Ca2+, Cu2+, Ag+ and Pb2+ in aqueous solutions. The OFF–ON fluorescence enhancement upon Hg2+ binding could be ascribed to conformational change of the spirolactam moiety of the rhodamine fluorophore through a spirolactam ring opening process. Furthermore, encapsulation of the sensors by polymeric membranes (PMMA) provided high selectivity and high sensitivity, especially for the sensors-encapsulated polymeric membrane RBH that showed an extremely low detection limit (0.2 ppb), which was 245 times lower than the sensor RBH in aqueous solution. In addition to fluorescence enhancement, the presence of Hg2+ also induced a noticeable color change from colorless to pink for the sensors dissolved in aqueous solution (10% v/v MeOH/water). The detection limits of sensor RBH and R6GH in both formats were in the range of 10−9–10−7 M of Hg2+ and were sufficient for on-site Hg2+ detection in the environmental and biological systems such as ground water, drinking water and edible fish. In particular, the extremely high sensitivity of the novel sensors-encapsulated polymeric membrane RBH could pave the way for development of a real time Hg2+ detection portable device.

Colorimetric and fluorescent sensing of a new FRET system via [5]helicene and rhodamine 6G for Hg2+ detection

A “visually colorimetric” and fluorometric sensor based on [5]helicene connected to rhodamine 6G via a hydrazide moiety, RG5H, was designed and prepared for the highly sensitive and selective detection of Hg2+. The sensor system operated through the FRET process from the [5]helicene energy donor to the ring-opened rhodamine 6G–Hg2+ complex acceptor. The sensor illustrated a very large Stokes shift up to 176 nm. In aqueous acetonitrile solutions, RG5H exhibited high selectivity and sensitivity toward Hg2+ in the presence of various competitive metal ions, such as Cu2+, Ag+, Zn2+, Pb2+, Ca2+, Cd2+, Co2+, Fe2+, Mg2+, Mn2+, Ba2+, Ni2+, Na+, Li+ and K+. The detection limit of RG5H for determination of Hg2+ was found to be 2.3 ppb. The binding of RG5H to Hg2+ was evident in both fluorescence enhancement as well as a colorimetric change in the sensor from greenish-yellow to orange, which could be easily observed by the naked eye.

Dual-Analyte Fluorescent Sensor Based on [5]Helicene Derivative with Super Large Stokes Shift for the Selective Determinations of Cu2+ or Zn2+ in Buffer Solutions and Its Application in a Living Cell

A new fluorescent sensor, M201-DPA, based on [5]helicene derivative was utilized as dual-analyte sensor for determination of Cu2+ or Zn2+ in different media and different emission wavelengths. The sensor could provide selective and bifunctional determination of Cu2+ in HEPES buffer containing Triton-X100 and Zn2+ in Tris buffer/methanol without interference from each other and other ions. In HEPES buffer, M201-DPA demonstrated the selective ON-OFF fluorescence quenching at 524 nm toward Cu2+. On the other hand, in Tris buffer/methanol, M201-DPA showed the selective OFF-ON fluorescence enhancement upon the addition of Zn2+, which was specified by the hypsochromic shift at 448 nm. Additionally, M201-DPA showed extremely large Stokes shifts up to ∼150 nm. By controlling the concentration of Zn2+ and Cu2+ in a living cell, the imaging of a HepG2 cellular system was performed, in which the fluorescence of M201-DPA in the blue channel was decreased upon addition of Cu2+ and was enhanced in UV channel upon addition of Zn2+. The detection limits of M201-DPA for Cu2+ and Zn2+ in buffer solutions were 5.6 and 3.8 ppb, respectively. Importantly, the Cu2+ and Zn2+ detection limits of the developed sensors were significantly lower than permitted Cu2+ and Zn2+ concentrations in drinking water as established by the U.S. EPA and WHO.

Novel Cu2+-specific “Turn-ON” fluorescent probe based on [5]helicene with very large Stokes shift and its potential application in living cells

A novel fluorescent sensor based on a [5]helicene dye bearing hydrazine (1) was synthesized through a simple reaction. The sensor was fully characterized by NMR, single crystal X-ray analysis, and HRMS spectrometry. Fluorescence technique was also employed to study the ability of 1 for the detection of Cu2+ among different ions. It was found that 1 exhibited a highly sensitive fluorescence response toward Cu2+ with a “Turn-ON” fluorescence enhancement and a very large Stokes shift of ∼183 nm. The sensor selectively bound to Cu2+ in the presence of various metal ions, such as Cd2+, Ag+, Pb2+, Zn2+, Ni2+, Hg2+, K+, Li+, Ba2+, Al3+, Fe2+, Co2+, Mn2+, Ca2+, Na+, and Mg2+ in aqueous acetonitrile solution. The Cu2+ detection limit was 2.6 ppb (3σ/slope), which was lower than the permissible concentration in drinking water determined by the US EPA and WHO. In addition, the sensor can enhance fluorescence from the intracellular area in the HepG2 cellular system. Hence, it has the potential for the detection of Cu2+ in biological samples.

Triple detection modes for Hg2+ sensing based on a NBD-fluorescent and colorimetric sensor and its potential in cell imaging

A new fluorescent sensor (DiNBD) was successfully synthesized; the sensor consisted of two 7-nitrobenzo-2-oxa-1,3-diazolyl (NBD) units that were covalently attached to 2-(4-(2-aminoethylthio)butylthio)ethanamine by a conventional two-step synthesis. It could serve as a colorimetric and fluorescent sensor for Hg2+ in aqueous acetonitrile solutions with high selectivity over other competitive metal ions. Upon Hg2+ titration, the sensor exhibited “ON–OFF” fluorescence quenching at 526 nm (λex 458 nm) and “OFF–ON” fluorescence enhancement at 560 nm (λex 530 nm), which were accompanied by a colorimetric change from yellow to red. The Hg2+ detection limit of the sensor was 0.7 ppb, which is lower than the permitted level of Hg2+ in drinking water regulated by US EPA and WHO. Additionally, the fluorescence quenching and fluorescence enhancement of the sensor in the presence of Hg2+ could provide a precise determination of Hg2+ concentration due to their self-calibration. Moreover, the DiNBD fluorescent sensor could be used to determine Hg2+ in real samples and living cells, which could widen the applications of the developed sensor.

Environmentally friendly method for determination of ammonia nitrogen in fertilisers and wastewaters based on flow injection-spectrophotometric detection using natural reagent from orchid flower

ABSTRACT Crude aqueous extract from the orchid ‘Dendrobium Sonia earsakul’ was utilised as a natural product reagent in flow injection analysis (FIA) incorporating a gas diffusion unit (GD) for the determination of ammonia nitrogen. Sample solution was injected into a NaOH donor stream to generate ammonia gas (NH3). In the GD unit, NH3 diffused across a PTFE gas-permeable membrane into the acceptor stream of the orchid extract. As the result, the aqueous orchid reagent became more alkaline and its colour changed from purple to green. The change in the colour of orchid acceptor correlated with the concentration of ammonia nitrogen in the sample and its absorbance monitored by a spectrophotometer at 600 nm. Ammonia nitrogen in chemical fertiliser samples and wastewater samples from agricultural fields were determined and reported as %N (w/w) and mg N L−1, respectively. For chemical fertilisers which contained high content of ammonia nitrogen, a flow rate of 1.0 mL min−1 and injection volume of 100 µL were used with a linear range of 5–40 mmol L−1 and detection limit of 2.12 mmol L−1. However, a higher sensitivity was required for wastewater samples having low ammonia nitrogen content. The flow rate was reduced to 0.3 mL min−1 and the injection volume increased to 1000 µL. As a result, detection limit of 0.76 mmol L−1 was achieved with linear range of 1–5 mmol L−1. The results of our method agreed well with that using the OPA method employing fluorescence detection.