Sang-Mok Chang

Chiral recognition of mandelic acid by l-phenylalanine-modified sensor using quartz crystal microbalance

This study presents a new method for the highly selective recognition of chiral mandelic acid (MA) using L-phenylalanine (L-Phe) as the selector. The proposed method is based on quartz crystal microbalance (QCM) detection, integrated with a vapor diffused molecular assembly (VDMA) reaction technique. The construction of the L-Phe-modified QCM sensor involved a two-step assembly procedure. The chiral recognizability of L- and D-MA on the L-Phe-modified surface was then examined using the VDMA method and QCM. The chiral discrimination factor between L- and D-MA detected by QCM, alpha(L-MA/D-MA), was found to be about 8. The VDMA selective sensing of L-MA on the L-Phe-modified surface was also confirmed by the contact angle measurements. The L-Phe-modified QCM sensor showed good stability and reusability. The present chiral recognition results suggest that L-phe is an excellent resolving agent for the resolution of chiral mandelic acid.

The principle and applications of piezoelectric crystal sensors

Abstract The principle, construction, and applications of piezoelectric crystal sensors as universal sensor are reviewed. A historical overview and basic piezoelectric crystal physics as well as design considerations for different sensor system are reviewed. Most of previous reviews were treated with gas phase application, but this review is focused mainly on the liquid phase application, such as monitoring the dynamic microrheology phenomena of liquid crystal, lipid thin films, and electrochemical polymerized polypyrrole film.

Monitoring changes in the viscoelastic properties of thin polymer films by the quartz crystal resonator

This paper shows that a quartz crystal resonator is applicable to the investigation of rheological property changes of polymer blend and polymer films by the simultaneous measurement of resonant frequency and resonant resistance of the quartz crystal resonator. Phase transition phenomena of PMMA/PVAc blend films were studied in the thermal cycles. Viscoelastic property changes of PMMA homopolymers including a certain amount of plasticizer and the dependence of molecular size in blend film on phase transition temperature were also investigated. Both the resonant frequency and resonant resistance of film coated quartz crystals changed critically at the glass transition temperature. The responses of the resonant resistance showed more clear change than those of the resonant frequency because the resonant resistance directly reflects the information of energy dissipation in the coated films. The responses in the thermal cycle were represented as diagrams of the resonant frequency versus the resonant resistance and dR/dF versus temperature for detailed explanations. These diagrams showed clear glass transition temperatures of the films in the thermal cycles.

Versatile method for chiral recognition by the quartz crystal microbalance: chiral mandelic acid as the detection model.

Chiral recognition is considered to be the most important, fundamental basis in the development of separation technology for chiral isomers in the pharmaceutical and biotechnology fields. However, the selective detection of individual enantiomers is still one of the most difficult analytical tasks because of the close similarity of the molecular configurations between chiral isomers. This study presents a versatile vapor-diffused molecular assembly (VDMA) reaction approach for chiral recognition by the quartz crystal microbalance (QCM). Chiral L/D-mandelic acid (MA) was used as the detection model, and L-phenylalanine (L-Phe) was used as the selector. The construction of the L-Phe-modified QCM sensor involved a four-step layer-by-layer assembly procedure. Each modification step was analyzed by cyclic voltammetry, the contact angle, and a resonance frequency measurement. The chiral recognizability of the L-Phe-modified QCM sensor to L-mandelic acid was then examined by resonance frequency measurement using the novel VDMA technique and also investigated by atomic force microscope (AFM) measurements. A chiral discrimination factor of up to approximately 9 between L- and d-MA on the L-Phe-modified QCM sensor was obtained by using this gaseous-phase reaction technique. AFM results also showed obvious selective aggregation of L-MA on the L-Phe-modified surface but no noticeable aggregation of D-MA during the VDMA reaction. Both of the QCM and AFM results confirmed the usefulness of this proposed VDMA technique for the study of chiral recognition. The main advantage of the proposed method is that it offers a universal simple application scheme for the QCM detection of small resonance frequency changes due to chiral molecular recognition by a chiral selector immobilized on the QCM sensor surface.

In Situ Optoelectrochemical Approach for the Dynamic Property Study of Polypyrrole Thin Film by Quartz Crystal Combined with UV‐Visible Advanced Design

We have designed a new experiment system composed of quartz crystal analyzer (QCA) and UV‐visible (UV‐vis) spectroscopy. As an example of an application, the redox process of polypyrrole was investigated and it was easy to understand the electrochemical mechanism occurring on the electrode surface. The QCA‐UV/vis system can provide plentiful analytical information for the study of electrode‐solution interface phenomena. In this paper we suggest eight analytical relations for investigating the electrochromic and viscoelastic property change due to electrochemical mass transport that occurs during the redox process of polypyrrole.

Stability study of carbon graphite covered quartz crystal

This paper shows the properties and the possibilities of carbon graphite-covered quartz crystal as an organic gas-sensing device. The carbon graphite formed on a side of quartz plate was supported by Pt electrodes. The prepared quartz crystals were heat treated for the enhancement of stability in the temperature range from 100 to 450°C. Impedance analysis of the carbon graphite-covered quartz crystal showed a larger admittance value than that of a lipid coated quartz crystal, which was based on our previous method for detecting gas components. Using the adsorption and desorption responses, the reproducibility and sensitivity are presented for describing the merits of carbon graphite-covered quartz crystal in the detection of organic gas components. The comparison of sensitivity between commercially available alcohol sensor and carbon graphite-covered quartz crystal is provided for a ppm of ethanol injection. These results prove that the carbon graphite-covered quartz crystals are usable as an organic gas sensor and as an analytical device.

Conventional detection of 2,4-dinitrophenol using quartz crystal microbalance

We present conventional detection of 2,4-dinitrophenol (DNP) for using the competitive reaction between DNP and DNP-conjugated albumin onto DNP antibody immobilized quartz crystal microbalance (QCM). This QCM method allows detection of DNP concentration in the range of 0.01 to 100 ng/ml; linear correlation obtains DNP concentration from 1 to 100 ng/ml.

DC electric-field-induced DNA stretching for AFM and SNOM studies

An effective method of DNA stretching on mica surfaces is proposed for an extremely low concentration of DNA. The method is based on an electric field and well applied on the concentration range from 57 x 10(-3) to 57 x 10(-6) ng/ml. The stretching exists in a gap between positive and negative electrodes. The difference in the stretching efficiency among the different surfaces of bare mica, Mg2+ soaked mica and AP-mica is discussed. The best performance of the stretching is found from the surface of AP-mica for the same experimental condition of sample concentration and applied voltage. Finally, from a Scanning near-field optical microscope image, it is found that well-stretched DNA molecules have shown more similar optical resolution, which is inferred from an optical fiber probe, itself.

Study on Mixing Condition of the Rubber Composite Containing Functionalized S-SBR, Silica and Silane : II. Effect of Mixing Temperature and Time

Department of Chemical Engineering, Dong-A Univ., 840 Hadan-dong, Saha-gu, Busan, 604-714, Korea(Received May 3, 2013, Revised May 10, 2013, Accepted May 21, 2013)요 약：실리카와 실란을 포함하는 고무 복합체의 최적 배합조건을 찾기 위하여 배합온도와 시간을 달리한 고무복합체의 특성을 평가하였다. 고무복합체의 제조를 위한 배합온도는 120 ℃, 140 ℃, 160 ℃의 다양한 배합시간으로 배합한 후 고무복합체의 특성을 평가하였다. 배합시간이 증가할수록 점도의 감소와 결합고무가 증가하였으나, 배합시간의 증가에 따른 점도의 변화는 배합온도가 낮을수록 크고, 결합고무의 증가속도는 온도가 높을수록 크게 나타났다. 낮은 온도인 120 ℃인 경우 배합 시간이 바뀌어도 가교 속도는 거의 일정한 반면 10분 이하의 배합 시간에서는 분산이나 반응이 충분치 않음을 동적 점탄성과 분산 평가 등을 통하여 알 수 있었다. 반대로 높은 온도에서는 충분한 반응에 의하여 동적 점탄성 특성 등이 우수한 것으로 나타나지만 가교 속도나 물성의 변화 등이 매우 민감하고 과도한 결합고무의 형성 때문에 적합한 온도 조건이 아님을 확인하였고, 변성 S-SBR 실리카 배합에서는 140 ℃에서 10분 정도의 배합이 분산과 실리카-실란 결합의 최적 조건임을 확인하였다. ABSTRACTThe properties of the rubber composites containing a silane and silica were evaluated by changing the mixing time and temperature, in order to find the optimum mixing conditions. Characteristics of the compounds were evaluated after mixing at 120 , 140 , and 160 with various mixing time. With increasing of mixing time, mooney viscosity decreased while the bound rubber contents of the compounds increased. Viscosity rise by increased mixing time was bigger at low temperature and the higher the mixing temperature the faster in the formation of bound rubber. With lower mixing temperature of 120 , cross-linking rate was almost constant. Dynamic viscoelastic properties and dispersity of the compound showed that dispersion of ingredients and reaction was not sufficient with the mixing time of less than 10min. On the contrary, with high temperature, it was obvious that good dynamic and physical properties could be obtained due to sufficient coupling reaction, however it was thought this high temperature is not optimum because of sensitive cross-linking rate and physical properties and excessive formation of bound rubber. Consequently, it was confirmed that the mixing condition of 10min at 140 was optimum for the silane coupling reaction and dispersion of functionalized S-SBR containing silica and silane.Keywordssolution SBR, silanization, mixing temperature, mixing time, dynamic viscoelastic properties

Quartz crystal microbalance technique for analysis of cooling crystallization.

A quartz crystal microbalance (QCM) technique is developed for the in situ analysis of the cooling crystallization processes of crystal nucleation and growth. In contrast to conventional techniques based on property changes in the solid or solution phase, the proposed QCM technique simultaneously exploits property changes in both the solid and solution phases, such as the solid mass and liquid viscosity, to analyze the crystallization processes. When initially cooling the solution, an increase in the solution viscosity is reflected in the QCM responses for the resonant frequency and resonant resistance. With further cooling, the resonant frequency and resonant resistance sharply change at the induction point of crystal nucleation, as the viscous liquid film on the sensor suddenly shifts to an elastic solid phase. Thereafter, the QCM responses are mainly controlled by the suspension viscosity due to simultaneous crystal nucleation and growth with further cooling. As a result, the QCM responses allow accurate measurement of the induction point and metastable zone width during the cooling crystallization. Additional mechanistic information on the crystallization, including molecular cluster formation, crystal nucleation, and crystal growth, is also extracted from a resonant frequency-resistance plot (F-R plot) of the QCM responses when varying the cooling conditions.