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ABSTRACT: Gold is known to have good biocompatibility because of its inert activity and the surface property can be easily tailored with self-assembled monolayers (SAMs). In previous works, gold surfaces were tailored with homogeneously mixed amine and carboxylic acid functional groups to generate surfaces with a series of isoelectronic points (IEPs). In other words, by tailoring the chemical composition in binary SAMs, different surface potentials can be obtained under controlled pH environments. To understand how the surface potentials affect the interaction at the interface, a binary-SAMs-modified Au electrode on a quartz crystal microbalance with dissipation detection (QCM-D) was used owing to the high weight sensitivity of QCM-D. In QCM-D, the frequency shift and the energy dissipation are monitored simultaneously to determine the adsorption behaviors of the plasmid DNA to surfaces of various potentials in Tris-buffered NaCl solutions of different pH. The results revealed that the plasmid DNA can be adsorbed on the SAM-modified surfaces electrostatically; thus, in general, the amount of adsorbed plasmid DNA decreased with increasing environmental pH and the decreasing ratio of the amine functional groups on the surfaces owing to weaker positive potentials on the surface. For the high amine-containing surfaces, due to the strong electrostatic attraction, denser films were observed, and thus, the apparent thickness decreased slightly. The negatively charged carboxylic acid surfaces can still adsorb the negatively charged plasmid DNA at some conditions. In other words, the electrostatic model cannot explain the adsorption behavior completely, and the induced dipole (Debye) interaction between the charged and polarizable molecules needs to be considered as well.
Journal of Colloid and Interface Science 06/2012; 382(1):97-104. · 3.07 Impact Factor
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ABSTRACT: Time-of-flight secondary ion mass spectrometry (ToF-SIMS) using pulsed C(60)(+) primary ions is a promising technique for analyzing biological specimens with high surface sensitivities. With molecular secondary ions of high masses, multiple molecules can be identified simultaneously without prior separation or isotope labeling. Previous reports using the C(60)(+) primary ion have been based on static-SIMS, which makes depth profiling complicated. Therefore, a dynamic-SIMS technique is reported here. Mixed peptides in the cryoprotectant trehalose were used as a model for evaluating the parameters that lead to the parallel detection and quantification of biomaterials. Trehalose was mixed separately with different concentrations of peptides. The peptide secondary ion intensities (normalized with respect to those of trehalose) were directly proportional to their concentration in the matrix (0.01-2.5 mol%). Quantification curves for each peptide were generated by plotting the percentage of peptides in trehalose versus the normalized SIMS intensities. Using these curves, the parallel detection, identification, and quantification of multiple peptides was achieved. Low energy Ar(+) was used to co-sputter and ionize the peptide-doped trehalose sample to suppress the carbon deposition associated with C(60)(+) bombardment, which suppressed the ion intensities during the depth profiling. This co-sputtering technique yielded steadier molecular ion intensities than when using a single C(60)(+) beam. In other words, co-sputtering is suitable for the depth profiling of thick specimens. In addition, the smoother surface generated by co-sputtering yielded greater depth resolution than C(60)(+) sputtering. Furthermore, because C(60)(+) is responsible for generating the molecular ions, the dosage of the auxiliary Ar(+) does not significantly affect the quantification curves.
Analytica chimica acta 03/2012; 718:64-9. · 4.31 Impact Factor
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Szu-Hsian Lee,
Wei-Chun Lin,
Chi-Jen Chang,
Chih-Chieh Huang,
Chi-Ping Liu,
Che-Hung Kuo,
Hsun-Yun Chang,
Yun-Wen You,
Wei-Lun Kao,
Guo-Ji Yen,
Ding-Yuan Kuo, Yu-Ting Kuo,
Meng-Hung Tsai,
Jing-Jong Shyue
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ABSTRACT: This study demonstrated that the work function (Φ) of Au substrates can be fine-tuned by using series ratios of binary self-assembled monolayers (SAMs). By using pure amine- and carboxylic acid-bearing alkanethiol SAM on gold substrates, Φ of Au changed from 5.10 to 5.16 and 5.83, respectively, as determined by ultra-violet photoelectron spectrometry (UPS). The shift in Φ due to the use of different functional groups was rationalized by considering the dipole moments of the molecules anchored on the Au surface. A series of binary SAMs were fabricated by mixing carboxylic acid- and amine-terminated alkanethiols in the deposition solution. By mixing these functional groups in SAMs, a linear correlation between Φ with respect to chemical composition (hence the effective dipole moment on the Au surface) was observed. It was found that arbitrary Φ between extremes (5.16 and 5.83) controlled by respective functional groups can be obtained by changing the chemical composition of SAMs. The Scanning Kelvin Probe (SKP) was also used to measure the contact potential difference (CPD) between SAMs and referencing Au on a patterned substrate prepared by photo-lithography. It was found that the CPD of SAMs with different chemical compositions correlates to their Φ. However, the magnitude of the CPD was smaller than the difference in Φ measured by UPS that was possibly due to the adsorption of contaminants in air.
Physical Chemistry Chemical Physics 03/2011; 13(10):4335-9. · 3.57 Impact Factor
