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A molecular-gap device for specific determination of mercury ions.

Guo Z, Liu ZG, Yao XZ, Zhang KS, Chen X, Liu JH, Huang XJ - Sci Rep (2013)

Bottom Line: Despite great success, many inevitably encounter the interferences from other metal ions besides the complicated procedures and sophisticated equipments.Notably, the fabricated molecular-gap device shows a specific response toward Hg(2+) with a low detection limit actually measured down to 1 nM.Theoretical calculations demonstrate that the specific sensing mechanism greatly depends on the electron transport ability of glutathione dimer bridged by heavy metal ions, which is determined by its frontier molecular orbital, not the binding energy.

View Article: PubMed Central - PubMed

Affiliation: Nanomaterials and Environmental Detection Laboratory, Hefei Institutes of Physical Science, Chinese Academy of Sciences, Hefei 230031, P. R. China.

ABSTRACT
Specific determination/monitoring of trace mercury ions (Hg(2+)) in environmental water is of significant importance for drinking safety. Complementarily to conventional inductively coupled plasma mass spectrometry and atomic emission/absorption spectroscopy, several methods, i.e., electrochemical, fluorescent, colorimetric, and surface enhanced Raman scattering approaches, have been developed recently. Despite great success, many inevitably encounter the interferences from other metal ions besides the complicated procedures and sophisticated equipments. Here we present a molecular-gap device for specific determination of trace Hg(2+) in both standardized solutions and environmental samples based on conductivity-modulated glutathione dimer. Through a self-assembling technique, a thin film of glutathione monolayer capped Au nanoparticles is introduced into 2.5 μm-gap-electrodes, forming numerous double molecular layer gaps. Notably, the fabricated molecular-gap device shows a specific response toward Hg(2+) with a low detection limit actually measured down to 1 nM. Theoretical calculations demonstrate that the specific sensing mechanism greatly depends on the electron transport ability of glutathione dimer bridged by heavy metal ions, which is determined by its frontier molecular orbital, not the binding energy.

No MeSH data available.


Comparative studies of the energies of the molecular orbitals of the complexes.(a) HOMO and LUMO energy gap of GSH dimer and complexes of GSH dimer bridged by Zn2+, Cd2+, Hg2+ and Pb2+. (b) HOMO and LUMO. The case before capture of cations (marked by GSH-GSH) was included for comparison.
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f4: Comparative studies of the energies of the molecular orbitals of the complexes.(a) HOMO and LUMO energy gap of GSH dimer and complexes of GSH dimer bridged by Zn2+, Cd2+, Hg2+ and Pb2+. (b) HOMO and LUMO. The case before capture of cations (marked by GSH-GSH) was included for comparison.

Mentions: According to previous reports about the electron transport of molecular junction, the conductivity of molecular mainly depends on its frontier orbital2343444546. Initiate from this view, frontier orbital of GSH dimer and its complexes with metal ions have been further investigated. The energies of their highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) are listed in Table S2 (More details can be seen in the Supplementary Information). The energy gap between of them is described in Fig. 4a. For the GSH dimer, its energy gap is about 6.17 eV. Once combined with metal ions, their energy gaps are changed. Bridged by Zn2+, it deceases to 5.76 eV. Similar effects can be seen with the binding of Cd2+, Hg2+ and Pb2+. It is about 4.89 eV for Cd2+ and 3.86 eV for Hg2+, indicating that the energy gap gradually decreases from top to bottom along the same group. However, linked by Pb2+, it is about 5.29 eV larger than that of the complex bridged by the same periodic Hg2+. From the Fig. 4b, it can be found that the HOMO orbitals are always located, which is closed to the connected Au NPs for GSH dimer before and after binding with heavy metal ions. For the LUMO orbital, it is mostly centered and separated at the centre of its dimer before binding. However, after binding it mainly focused on metal atoms. To realize the electron flowing between two Au NPs, the electrons should be injected from the HOMO into the LUMO. Combined with the analysis of HOMO and LUMO for GSH dimer and their complexes, possible specific sensing mechanism has been offered in the following. The electron transport process is illustrated in Fig. 5, which shows an idealized band diagram. The energy (Ea) at which the majority of tunneling occurs is above the Fermi energy (Ef) but below the LUMO. Under a given applied bias voltage, the electron transport between Au NPs mainly depends on the thermally assisted tunneling effect for the model a, in which the LUMO (−0.41 eV) of GSH dimer between Au NPs will be higher than Ea without binding with metal ions. After binding with Zn2+, Cd2+ and Pb2+, their energies of LUMO are decreased to −0.76, −1.62 and −1.22 eV, respectively. Maybe they are still higher than Ea, as shown in the model b. However, for Hg2+ it is down to −2.65 eV, which may be close or lower than Ea. Furthermore, the energy gap of HOMO and LUMO is down to 3.86 eV, which is greatly lower than those of GSH dimer binding with other metal ions. Accordingly, electrons should be easily injected from the HOMO into the LUMO, leading to the enhancement of electron transport in the model c.


A molecular-gap device for specific determination of mercury ions.

Guo Z, Liu ZG, Yao XZ, Zhang KS, Chen X, Liu JH, Huang XJ - Sci Rep (2013)

Comparative studies of the energies of the molecular orbitals of the complexes.(a) HOMO and LUMO energy gap of GSH dimer and complexes of GSH dimer bridged by Zn2+, Cd2+, Hg2+ and Pb2+. (b) HOMO and LUMO. The case before capture of cations (marked by GSH-GSH) was included for comparison.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC3814579&req=5

f4: Comparative studies of the energies of the molecular orbitals of the complexes.(a) HOMO and LUMO energy gap of GSH dimer and complexes of GSH dimer bridged by Zn2+, Cd2+, Hg2+ and Pb2+. (b) HOMO and LUMO. The case before capture of cations (marked by GSH-GSH) was included for comparison.
Mentions: According to previous reports about the electron transport of molecular junction, the conductivity of molecular mainly depends on its frontier orbital2343444546. Initiate from this view, frontier orbital of GSH dimer and its complexes with metal ions have been further investigated. The energies of their highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) are listed in Table S2 (More details can be seen in the Supplementary Information). The energy gap between of them is described in Fig. 4a. For the GSH dimer, its energy gap is about 6.17 eV. Once combined with metal ions, their energy gaps are changed. Bridged by Zn2+, it deceases to 5.76 eV. Similar effects can be seen with the binding of Cd2+, Hg2+ and Pb2+. It is about 4.89 eV for Cd2+ and 3.86 eV for Hg2+, indicating that the energy gap gradually decreases from top to bottom along the same group. However, linked by Pb2+, it is about 5.29 eV larger than that of the complex bridged by the same periodic Hg2+. From the Fig. 4b, it can be found that the HOMO orbitals are always located, which is closed to the connected Au NPs for GSH dimer before and after binding with heavy metal ions. For the LUMO orbital, it is mostly centered and separated at the centre of its dimer before binding. However, after binding it mainly focused on metal atoms. To realize the electron flowing between two Au NPs, the electrons should be injected from the HOMO into the LUMO. Combined with the analysis of HOMO and LUMO for GSH dimer and their complexes, possible specific sensing mechanism has been offered in the following. The electron transport process is illustrated in Fig. 5, which shows an idealized band diagram. The energy (Ea) at which the majority of tunneling occurs is above the Fermi energy (Ef) but below the LUMO. Under a given applied bias voltage, the electron transport between Au NPs mainly depends on the thermally assisted tunneling effect for the model a, in which the LUMO (−0.41 eV) of GSH dimer between Au NPs will be higher than Ea without binding with metal ions. After binding with Zn2+, Cd2+ and Pb2+, their energies of LUMO are decreased to −0.76, −1.62 and −1.22 eV, respectively. Maybe they are still higher than Ea, as shown in the model b. However, for Hg2+ it is down to −2.65 eV, which may be close or lower than Ea. Furthermore, the energy gap of HOMO and LUMO is down to 3.86 eV, which is greatly lower than those of GSH dimer binding with other metal ions. Accordingly, electrons should be easily injected from the HOMO into the LUMO, leading to the enhancement of electron transport in the model c.

Bottom Line: Despite great success, many inevitably encounter the interferences from other metal ions besides the complicated procedures and sophisticated equipments.Notably, the fabricated molecular-gap device shows a specific response toward Hg(2+) with a low detection limit actually measured down to 1 nM.Theoretical calculations demonstrate that the specific sensing mechanism greatly depends on the electron transport ability of glutathione dimer bridged by heavy metal ions, which is determined by its frontier molecular orbital, not the binding energy.

View Article: PubMed Central - PubMed

Affiliation: Nanomaterials and Environmental Detection Laboratory, Hefei Institutes of Physical Science, Chinese Academy of Sciences, Hefei 230031, P. R. China.

ABSTRACT
Specific determination/monitoring of trace mercury ions (Hg(2+)) in environmental water is of significant importance for drinking safety. Complementarily to conventional inductively coupled plasma mass spectrometry and atomic emission/absorption spectroscopy, several methods, i.e., electrochemical, fluorescent, colorimetric, and surface enhanced Raman scattering approaches, have been developed recently. Despite great success, many inevitably encounter the interferences from other metal ions besides the complicated procedures and sophisticated equipments. Here we present a molecular-gap device for specific determination of trace Hg(2+) in both standardized solutions and environmental samples based on conductivity-modulated glutathione dimer. Through a self-assembling technique, a thin film of glutathione monolayer capped Au nanoparticles is introduced into 2.5 μm-gap-electrodes, forming numerous double molecular layer gaps. Notably, the fabricated molecular-gap device shows a specific response toward Hg(2+) with a low detection limit actually measured down to 1 nM. Theoretical calculations demonstrate that the specific sensing mechanism greatly depends on the electron transport ability of glutathione dimer bridged by heavy metal ions, which is determined by its frontier molecular orbital, not the binding energy.

No MeSH data available.