Time-dependent pH sensing phenomena using CdSe/ZnS quantum dots in EIS structure
© Kumar et al.; licensee Springer. 2014
Received: 20 February 2014
Accepted: 31 March 2014
Published: 12 April 2014
Time-dependent pH sensing phenomena of the core-shell CdSe/ZnS quantum dot (QD) sensors in EIS (electrolyte insulator semiconductor) structure have been investigated for the first time. The quantum dots are immobilized by chaperonin GroEL protein, which are observed by both atomic force microscope and scanning electron microscope. The diameter of one QD is approximately 6.5 nm. The QDs are not oxidized over a long time and core-shell CdSe/ZnS are confirmed by X-ray photon spectroscopy. The sensors are studied for sensing of hydrogen ions concentration in different buffer solutions at broad pH range of 2 to 12. The QD sensors show improved sensitivity (38 to 55 mV/pH) as compared to bare SiO2 sensor (36 to 23 mV/pH) with time period of 0 to 24 months, owing to the reduction of defects in the QDs. Therefore, the differential sensitivity of the QD sensors with respect to the bare SiO2 sensors is improved from 2 to 32 mV/pH for the time period of 0 to 24 months. After 24 months, the sensitivity of the QD sensors is close to ideal Nernstian response with good linearity of 99.96%. Stability and repeatability of the QD sensors show low drift (10 mV for 10 cycles) as well as small hysteresis characteristics (<10 mV). This QD sensor is very useful for future human disease diagnostics.
Among the numerous chemical sensors, pH sensor is the major field of research area, which is one of the controlled parameter for the biochemical industrial processes. Lots of aspects have been identified to detect the hydrogen ions under different environment conditions. In development of solid state sensor, recent approaches are ISFET (ion-sensitive field effect transistor), LAPS (light addressable potentiometric sensor), and capacitance-based electrolyte insulator semiconductor (EIS) [1–4]. Among these developments, EIS has shown potential in terms of its simple structure, label-free detection, easy fabrication procedure, and cost effectiveness [5, 6]. In addition, nanoparticles have generated considerable interest as diagnostic tool because of their small sizes and comparatively higher surface area that leads to more interaction with ions in solution [7–10]. Semiconductor nanoparticles such as quantum dots (QDs) are one of the major candidates being studied for sensor development [11, 12]. The QDs are better than bare SiO2 sensing membrane because of their high surface area to volume ratio which gives the platform for controlled immobilization of the biomolecules. In addition, the QDs have been studied as fluorescent labels for bioimaging as well as ionic probes to detect chemical ion concentration in electrolyte solution and immunosensor for cancer detection [13–16]. Long-term environmental stability for robust sensing device is still a major limitation due to environmental factors, such as exposure of reactive ions, humidity, and temperature; results in transformation of nanoparticles such as photooxidation or size change have been reported earlier [17–20]. The controlled distribution of QDs to prevent agglomeration on sensing surface is another important aspect for sensitivity enhancement as well as long-term stability of the device. Some protein-mediated approaches have been demonstrated for the controlled ordering of quantum dots array [21–23]. Chaperonin GroEL protein can be used as template to deposit the quantum dots on sensing membrane as they self-assembled themselves in two-dimensional (2D) array. Although many researchers have reported the sensing properties using different nanoparticles, time-dependent improved pH sensitivity using CdSe/ZnS QDs has not yet been reported.
In this study, time-dependent pH sensing behavior of CdSe/ZnS QD membrane on SiO2/Si in EIS structure has been investigated for the first time. The QDs embedded in protein are observed by both atomic force microscope (AFM) and field-emission scanning electron microscope (FE-SEM) images. After annealing at 300°C, the QDs can be observed clearly by SEM due to the removal of protein. The chemical states of the core-shell QDs have been investigated by x-ray photoelectron spectroscopy (XPS). It is found that the QDs are not oxidized, however, water adsorption from environment can be the factor, which results lower defects in the QDs' surface. The values of sensitivity are approximately 34 and 55 mV/pH after initial and 24 months, respectively. The values of differential sensitivity of the QD with respect to bare SiO2 sensors are improved from 12 to 32 mV/pH for longer time, owing to higher surface states of the QDs. A good pH sensing linearity of 99.96% is also obtained with QDs-modified sensor.
The surface topography of chaperonin mediated QDs distribution on SiO2 surface was investigated by using an Innova scanning probe microscope (SPM) system (Bruker Corp., Bellerica, MA, USA). The AFM image was measured in tapping mode with a scan at area of 500 × 500 nm2. The size and topography of the QDs were investigated using FE-SEM (MSSCORPS Co. Ltd., Taiwan). The chemical bonding of the CdSe and ZnS elements was investigated by XPS. The EIS structure was transferred to the analyzing chamber at ultra-high vacuum of 1 × 10-9 Torr. The XPS spectra were recorded using Al Ka monochromatic x-ray source with energy of 1,486.6 eV. The scan was from 0 to 1,350 eV with step energy of 1 eV.
Capacitance-voltage (C-V) measurement was done using HP4284A in different pH buffer solutions. An Ag/AgCl electrode was used as a reference electrode and it was grounded during C-V measurement. The bias was applied on the Al bottom electrode. All measurements were done at 100 Hz. To obtain the steady results, all samples were kept in reverse osmosis (RO) water for 24 h before measurement. The EIS sensors were washed with deionized (DI) water before electrode transfer to subsequent pH solution. The ConCap response of QD-modified EIS pH sensor was measured up to ten repeated cycles to check any drift in pH sensitivity in each buffer solution. ConCap response was studied from acidic to basic pH and reversed to study the hysteresis effect of EIS sensors. To measure ConCap response, the QD-modified EIS sensor was washed with DI water after each step during repetitive measurement at the same buffer solution.
Results and discussion
where E is the sensing membrane potential without electrolyte solution, R is the universal gas constant of 8.314 JK-1 mol-1. T is the absolute temperature, and F is Faraday constant of 9.648 × 10-4C-mol-1. It is assumed that the CdSe/ZnS QDs immobilized at SiO2 surface have higher negative charge results in the thicker stern layer or more H+ ion accumulation at sensor-electrolyte interface results in higher density of ionic states at the surface. The higher ions' reactivity at the sensing membrane surface lead to higher surface potential which ultimately results in more Vfb shift for the QD sensors.
It is interesting to note that the time-dependent sensitivity of both the EIS sensors is observed over a time period of 24 months. A comparison of the sensitivity and linearity study of bare SiO2 and CdSe/ZnS quantum dot sensors at different time periods is shown in Figure 7. Initially, the bare SiO2 sensors show the pH sensitivity 35.87 mV/pH with linearity 97.26%. The sensitivity of bare SiO2 EIS sensors is not stable and even worse with time (Figure 7a). The values of sensitivities (linearity) are found to be 26 (97.28%) and 23 mV/pH (98.24%) after 12 and 24 months, respectively. The degradation in sensitivity of bare SiO2 EIS sensor with time is attributed to the dissolution of silanol at higher acidic or basic pH in electrolyte solution. On the other hand, the sensitivity of the QD sensors shows stable and better response than the bare SiO2 sensors. Initially, the CdSe/ZnS QD sensors show the sensitivity of 38.3 mV/pH with good linearity of 99.40% (Figure 7b), which is comparatively higher than the pH sensing response of Au nanoparticles as reported by Gun et al. . The values of sensitivity are improved to 52.5 and 54.7 mV/pH, while the values of linearity are found to be 99.92% and 99.96% after 12 and 24 months, respectively. After 24 months, the sensitivity of the QD sensors is near to ideal Nernstian response. The differential sensitivity of the QD with respect the bare SiO2 sensors also remarkably improved from 2 to 32 mV/pH with time. Therefore, the QD sensor can be used as a differential sensor. Cordero et al.  proposed the improved luminescence behavior of QDs after passivation of the surface trap states by adsorption of water molecules and reduction in the defect sites at CdSe quantum dots. However, this phenomenon is followed by photooxidation of the QDs' surface, which is opposite of surface passivation, which induces the defects in QDs' surface. In our case, we observe the similar behavior over long time. The passivation of quantum dots' surface by water molecule adsorption is expected from the environment's humidity, as sensor devices were kept at room temperature and measured for pH sensitivity repeatedly. In addition, sensitivity evolution with time is also in agreement of mechanism proposed by Asami et al. . They reported the change in adsorption state of TOPO on CdSe surface as TOPO (Lewis base) passivates the unbonded Se surface on longer photoillumination, and the shift in adsorption state of TOPO leads to the change in surface states of CdSe nanocrystals.
The CdSe/ZnS QDs in EIS structure have been successfully immobilized on SiO2 film using chaperonin protein. The QDs are observed by AFM and FE-SEM images, and the diameter of each QD is found to be approximately 6.5 nm. The core-shell CdSe/ZnS QDs are also confirmed by XPS, and the QDs are not oxidized even after long exposure time in air. Initially, improved pH sensitivity of the QD sensor is observed as compared to the bare SiO2 sensor (approximately 38 vs. 36 mV/pH) and it is further improved after 24 months (approximately 55 vs. 23 mV/pH), and the differential sensitivity with respect to bare SiO2 sensor is improved from 2 to 32 mV/pH, owing to the reduced defects in QDs with time. Good linearity of 99.96% is also obtained for a longer time. In addition, good stability and repeatability of quantum dots-modified EIS sensors are obtained by ConCap response of devices at 2 to 12 pH buffer solutions. This simple QD EIS sensor paves a way in future human disease investigation.
This work was also supported by the National Science Council (NSC), Taiwan.
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