|
CHAPTER I
Introduction
1.1 History and background of ISFET
The concentration of biologically relevant ions is a very important indicator associated with the state of health for human disease prediction. In numerous ions, the detection of hydrogen ion and potassium ion concentrations is of interest to bio-medical research. The changes in the potassium ion concentration in human serum increases the risk of acute cardiac arrhythmia [1] and changes in the hydrogen ion in urine increases the risk of kidney failure [2]. Furthermore, pH sensor could be used to detect the variation of pH caused from enzymatic reaction as a bio-sensing element [3-5]. Besides the aforementioned applications in the bio-medical field, the measurement of the potassium ion and hydrogen ion concentrations is also important for food and wine quality testing and water pollution monitoring in industrial and environmental fields [6-8]. Therefore, the development of sensors for continuous monitoring in real-time and in-line is required for these applications.
In recent years, many researches have been done to characterize ion sensitive field-effect transistor (ISFET) based on MOS technology due to their wide applications on chemical and biomedical sensors [9-10]. The first ISFET with SiO2 insulator as sensing membrane for bio-medical applications was invented by Bergveld in 1970 [11]. The basic structure of ISFET was derived from metal-insulator-semiconductor field-effect transistor (MISFET) with the gate contact replaced by a reference electrodes and an electrolyte. With binding of ions on sensing membrane surface sites in an electrolyte, pH dependent surface potential change can be observed by the shift of threshold voltage. The supporting sensing mechanism of surface binding is called site-dissociation model and the schematic diagram of site-dissociation model is presented as shown in Fig. 1-1. All the dielectrics of the surface are containing a certain amount of hydroxyl groups from the surface density, which is a particular characteristic for any specific layer. Furthermore, when the surface is brought in contact with an electrolyte solution, it is assumed that these surface sites act as discrete sites for chemical reactions at the surface. Each surface site can be neutral, act as a proton donor or as a proton acceptor. Equation 1-1 and Equation 1-2 are the acidic reaction and basic reaction between the A-OH sites at the surface and the H+ ions in the bulk solution, respectively. The insulator surface included A-O-, A-OH, and A-OH2+ sites which were represented negative, neutral and positive surface sites, respectively. The equilibrium constants, Ka and Kb are the acidic and basic characters of the neutral site A-OH, respectively.
1-1
1-2
1-3
1-4
The relationship between ψ0 and pH can be realized from the Boltzmann equation in Equation 1-5:
1-5
where q is elementary charge, k is Boltzmann’s constant, T is temperature and ψ0 is the potential difference between the surface and bulk of the solution. The surface potential is generated by the net surface charge, σ0.
1-6
In Equation 1-6, the total number of sites per unit area is presented in Equation 1-7:
1-7
The relationship between [H+]b, ψ0, and σ0 can be derived from Equation 1-3 to Equation 1-7 in terms of Ka, Kb and Ns in Equation 1-8, which are the characterizing parameters of a specific oxide.
1-8
To derived the relationship between pH=-log10[H+]b and ψ0, σ0 should be expressed in terms of ψ0, and in the Equation 1-8, 2(KaKb)1/2<< 1 and σ0 << qNs. In practical cases, the double layer capacitance can be as the simple constant capacitance of the Gouy-Chapman-Stern model, CDL, in which gives the relation between σ0 and ψ0 in Equation 1-9.
1-9
If we introduce as a reference point on the pH-scale and the pH value, ψ0 = 0, Equation 1-10 follows from Equation 1-8.
1-10
For the reason of ψ0 = 0, σ0 = 0, pHpzc means the pH at the point of zero charge, which gives the value of hydrogen ion concentration results in an electrically neutral surface. The final equation can be written as follows:
1-11
Where
1-12
The only parameter, β, determines the ψ0/pH relation. β is correlated with Ka, Kb and Ns and depends on the surface reactivity. It can be seen that the larger β, the pH vale behaves Nernstain response with a maximum sensitivity of 59 mV per decade.
Above the mentioned, the type of ISFET will be referred to as the electrolyte-insulator-semiconductor (EIS) field-effect transistor (EISFET) [12]. The current-voltage of EISFET is depend on solution and describes the EIS system. EIS is the center of ISFET and similar with MOS capacitor in MOSFET [13-14], the EIS structure was shown in Figure 1-2. Ion sensitivity is depended on a relationship between charge and potential at the insulator-electron surface of ideal EIS structure involved in EISFET. The schematic of site binding model can be extended to describe the experimental results of flat-band voltage determination from C-V measurements. The voltage shift of C-V curves of an EIS structure is from reference electrode (ERef) and the electrolyte-insulator interface (φ0). φ0 can be defined in Equation 1-13.
1-13
ψ0 means the pH-dependent surface potential and χsol is the surface dipole potential of the solution. The flat-band voltage of an EIS structure can be expressed as Equation 1-14.
1-14
Except for ψ0 which depended on pH, all terms in this equilibrium equation are constants. C-V curves corresponding from EIS structures can be used as a relative measure of ψ0. For the convenient of EIS fabrication process and the minimum deviation compare with ISFET, EIS structure is suitable to study the sensing properties of sensing layers. In this study, EIS structure was used to investigate the sensing properties of HfO2 sensing membrane and event extended to add enzyme layer to develop biosensor based on EIS structure, which is shown in Figure 1-3.
To get higher pH-sensitivity and better stability, many kinds of insulators have been studied as pH sensitive gate insulator materials in past years, such as silicon nitride (Si3N4) [15], tin oxide (SnO2) [16], tantalum pentoxide (Ta2O5) [17] and so on. However, there are still some problems on these sensing materials, such as surface oxidation for Si3N4, low corrosion-resistant for Al2O3, and light-induced drift for Ta2O5. Recently, hafnium oxide (HfO2) which owns higher dielectric constant, better thermal stability and compatible process with CMOS technology was proposed as the promising gate dielectric for future shrinking generation. In our previous works, HfO2 with high sensitivity, low drift and small body effect was also proposed as the promising sensing material for pH detection [18-21]. However, for multiple ion sensing applications in the bio-medical field, the unmodified HfO2 sensing membrane exhibits low capability due to low sensitivity for the detection of potassium ion. In past years, many methods have been proposed for the fabrication of membranes for potassium ion detection, such as polymeric [22], ion-implanted [23], or electro-chemically sensitive layers [24]. However, some instability problems can occur with these methods, including poor adhesion when adding an extra electrochemically or polymerically sensitive membrane on an insulator surface, surface damage by implantation and a lifetime limitation for all methods. Therefore, to avoid the limitation above mention, the composed material without post processing is proposed as one of the promising methods for alkali ion sensing membrane fabrication. This composed material could have the advantages from the composited sensing materials and eliminate the disadvantages of each composited sensing material.
1.2 Automatic Layer Deposition (ALD) system
Recently, to have a smooth and uniform surface, atomic layer deposition (ALD) system was used for insulator deposition. About the film deposition by automatic layer deposition (ALD) were proposed. ALD is one of the techniques for depositing smooth has performed particularly well in aspects of semiconductor processes, such deposition of high-k materials for MOSFET and DRAM capacitors [25] amd metal deposition for interconnects. ALD is a pulse technique which growth film alternately and separately supplied gaseous precursors, and takes places through self-limiting surface reactions [26]. The advantages of accurate and simple thickness control, good conformity, capability to produce sharp interfaces and multilayer structures, large area capability, and so on.
In a “standard” CVD process a wafer or a group in a vacuum chamber where chemical vapors are thermally reacted at low pressure to deposit a film on the wafer. The deposition process is continuous-the vapors flow depends on the temperature, pressure, gas flow volumes and uniformity, chemical depletion effects and time. Controlling all of these parameters to the level required for good thickness control of thin films is very difficult.
ALD reactions are typically carried out in the 200℃ to 400 ℃ temperature range. The processing temperature range for ALD [27] is the region of nearly ideal ALD behavior between the non-ideal regions as shown in Figure 1-4. If the deposition temperature is too high, chemical bonding cannot be sustained or the density of chemically reactive sites is reduced- reducing deposition rates. If the deposition temperature is too low thermally activated chemisorption and film forming reaction rates decrease reducing deposition rates. As the deposition temperature is increased from low to high deposition, the deposition rate increases-reaches a peak and then decreases. The temperature window for maximum deposition rate is relatively wide compared to CVD processes that are much more temperature sensitive.
Precursors must be volatile and thermally stable to ensure efficient transportation so that reactions will not be precursors transportation controlled. The vapor pressure of precursors must be high enough to completely fill the deposition chamber so that monolayer deposition takes place within a reasonable length of time (approximate vapor pressure of 0.1 torr). Precursors must chemisorb onto the surface or rapidly react with surface groups and react aggressively with each other to keep deposition times short.
There four main types of ALD reactors:
Closed system chambers - the reaction chamber walls are designed to affect the transport of the precursors.
Open system chambers – the chamber is designed so that the walls do not affect the precursor transport – the chamber is very large relative to the wafer position.
Semi-closed systems chambers – a channel is formed by two wafers and the precursor is fed between the two wafers.
Semi-open systems chambers – similar to a semi-open system expect one side of the precursor channel is a wafer and the other side is gas limited.
In this work, HfO2 was deposited by semi-closed systems chambers and used as a sensing membrane of a urea biosensor.
1.3 Motivation
Although there are many applications about ISFET and EIS structure with various organic or inorganic sensing membranes for ions detection, the non-ideal effects still need to be improved and the unfavorable effects need to be solved. In this study, the composed materials as sensing membranes were used to change the characteristics of the pure material and improve non-ideal effect such as light affect, hysteresis, and so on.
Furthermore, depend on the good pH sensitivity of hafnium-doioxide sensing membrane by ALD system, the detection can be extended to urea detection which detect through the pH change resulted from urea hydrolyzed. Except traditional chemical solution of enzyme immobilization, plasmas treatment to replace some steps of the traditional chemical solution steps was investigated.
Fig.1-1 Illustration of site binding model
Fig.1-2 Schematic cross-section view of EIS structure
(b)
Fig. 1-3 Schematic cross-section view of enzyme-EIS structure
Fig. 1-4 Acceptable temperature window of ALD process
CHAPTER II
pH and pK detection of HfO2, HfxWyOz and WO3 sensing membranes based on EIS structure
2.1 Background of HfO2, HfxWyOz and WO3 sensing membranes
In recent publications, hafnium oxide (HfO2) with high sensitivity, low drift and compatible process with CMOS technology was proposed as the promising sensing material for pH detection [28-29]. However, for multiple ion sensing application in biomedical region, the pure HfO2 sensing membrane exhibits low capability due to low sensitivity for the detection of K+ ion. In past years, there are many different types of methods to modify the sensitivity or selectivity for K+ ion, such as polymeric, ion-implanted, or electro-chemically sensitive layers. However, some instable problems are in these published methods including the poor adhesion in adding extra electrochemically or polymeric sensitive membrane on insulator surface, surface damage by implantation and lifetime limitation in all methods.
The composed material without post processing is more suitable for alkali metal ion sensor fabrication. The objective of the presented study is to develop an inorganic pK sensor with high pH-sensitivity and low light effect based on EIS structure with HfO2-WO3 double-oxide thin films by co-sputtering technology. The published result reveals that the use of a metal oxide with a high pHpzc value is effective in reducing alkaline metal ions sensitivity [30]. Therefore, the WO3 thin film with the possibility for alkali metal ions detection was chosen as the composited material due to the low pHpzc value [31]. The pH and pK sensing properties including sensitivity, linearity, light effect, and hysteresis were all investigated in this article.
2.2 Experimental
2.2.1 HfO2, HfxWyOz and WO3 - EIS fabrication
In this work, the EIS structures were fabricated to investigate the pH-sensing properties of HfO2, HfxWyOz and WO3 membranes. Figure 2-1 shows the schematic of EIS structure. For the processing of EIS structures, the p-type (100) silicon wafers were used as a starting substrate after standard RCA cleaning. Then, a 500 Å thick silicon dioxide was grown in dry oxidation. After that, the 500 Å thick of HfO2, HfxWyOz and WO3 sensing membranes were deposited by reactive radio frequency (r.f.) co-sputtering technology with hafnium target and tungsten target at the same processing step. The purity of both two targets is 99.99 %. The gas flow ratio of O2/Ar was set to 0.25 and the total flow gas was controlled constant at 25 sccm. The rf power were 30 watt and 50 watt of tungsten target and 150watt of hafnium target for different composed HfxWyOz sensing membrane.
The r.f. sputter system was initially pumped down to 8×10-6 Torr and the pressure during processing maintained at 2×10-2 Torr. The detail of sputtering parameters, deposition rate and sample number in different compositions of HfxWyOz sensing membrane were shown in Table 1. Afterwards, a 3000 Å thick Al film was evaporated on backside of the silicon wafer to form the ohmic contact. SU8, a negative photo-sensitive epoxy was used to define sensing area by photolithography process. Final, EIS structures were assembled on copper line of printed circuit board (PCB) by silver gel to form conductive line. To package the samples, a hand-made epoxy was used to encapsulate the EIS structure and Cu line of print circuit board.
2.2.2 Test solutions preparation
To investigate the potassium ion sensing properties, the 5 mM Tris/HCl solution was prepared as buffer electrolyte which the pH value was kept at 8.5. The concentrations of potassium ion in a range between 10-5 M and 10-1 M were controlled with injecting 0.1 M KCl/Tris-HCl into buffer electrolyte by micropipette. The standard pH buffer solutions (Merk Inc.) from pH 2 to pH 12 and potassium buffer solution from pK 1 to pK 5 were all measured by the pH meter before measurement for buffer solution condition check and calibration.
2.2.3 Measurement system
To extract the pH and pK sensing properties, the capacitance-voltage (C-V) curves for EIS structures were measured by using HP4284A high precision LCR meter. The ac signal frequency was chosen at 500 Hz for all EIS samples. The gate voltages were applied to silver/silver chloride reference electrode for EIS structures. To obtain the steady pH response, all EIS samples were immersed in the reversed osmosis (RO) water for 12 h before measurement. All measurement setup were carried out in a Faraday cage at room temperature to keep from light and noise interference. The responsive voltages for all EIS samples were calculated with 0.6 Cmax. The actual pH values of standard buffer solutions were measured by pH meter before and after measurement for pH sensitivity calculation.
To investigate the hysteresis effect of HfO2, HfxWyOz and WO3 membranes, the measurements in various pH buffer solutions were performed in the following sequence of steps: pH 6.99-4-6.99-9.81-6.99 .The immersion time of each pH solution was 10 min, and the C-V measurements were performed at 0, 2, 4, 6 and 8 min. Each C-V measurement took around 90 s.
To investigate the drift effect of HfO2, HfxWyOz and WO3 membranes, the measurements in pH4 buffer solutions were performed for 12 hrs, each C-V cure was measure between 10 min.
2.3 Sensing characterization of HfO2, HfxWyOz and WO3 -–EIS structures
2.3.1 pH detection
In Figure 2-2, the C-V curves measured from the pH 2 to the pH 12 buffer solutions of the HfO2 and WO3 membrane were obtained. HfO2 membrane exhibited higher pH sensitivity (50.9 mV/pH) for pH ranging between 2 and 12 than WO3 membrane (32.8 mV/pH). For WO3 membrane, there are two obvious linear region of pH sensing. Small pH sensitivity obtained from pH 2 to pH 6 was 16.4 mV/pH and large pH sensitivity obtained from pH 8 to pH 12 was 41.2 mV/pH. It indicated that the WO3 membrane was not suitable for applying in acid solution.
Figure 2-3 shows the calculated pH-sensitivity of HfxWyOz sensing membranes with different composited conditions, the power ratio of Hf to W, 0/150, 30/150, 50/150 and 150/0. An increasing of pH-sensitivity from pH 2 to pH 12 was observed with increasing the ratio of HfO2 incorporation. The pH-sensitivity of WO3 (sample I), HfxWyOz (sample III), HfxWyOz (sample III), and HfO2 (sample IV) sensing membranes are 35.47, 42.01, 46.22, and 53.33 mV/pH form pH 2 to pH 12, respectively. The linearity of WO3 (sample I), HfxWyOz (sample III), HfxWyOz (sample III), and HfO2 (sample IV) sensing membranes are 98.11, 98.48, 99.87, and 99.9% form pH 2 to pH 12, respectively.
For WO3 membrane, small pH-sensitivity calculated from pH 2 to pH 6 was 20.02 mV/ pH with linearity of 98.84% and large pH-sensitivity calculated from pH 6 to pH 12 was 46.28 mV/ pH with linearity of 99.51%. It indicated that the WO3 membrane was not suitable for applying in acid solution. However, the WO3 membrane with HfO2 incorporation exhibited high pH-sensitivity for hydrogen ion detection in pH range between 2 and 12.
2.3.2 pK detection
Figure 2-4 shows the change in response voltage as a function of potassium ion concentration of HfxWyOz sensing membranes with different composited conditions of HfO2 and WO3. The ability of potassium ion sensing of HfO2-based sensing membranes was increasing a lot within high concentration range between 1 mM to 100 mM with increasing the ratio of WO3 incorporation. The reason is probable the low pHpzc value of WO3 membrane [31]. The calculated pK-sensitivity and linearity of HfxWyOz sensing membranes with different composited conditions, WO3 (sample I), HfxWyOz (sample III), HfxWyOz (sample III), and HfO2 (sample IV) were shown in Fig. 2-5. The pK-sensitivity of WO3 (sample I), HfxWyOz (sample III), HfxWyOz (sample III), and HfO2 (sample IV) sensing membranes are 21.49, 26.84, 18.68, and 7.07 mV/pK in the concentration range between 1 mM and 100 mM, respectively. The linearity of HfxWyOz (sample III), HfxWyOz (sample III), and HfO2 (sample IV) in the concentration range between 1 mM and 100 mM are all above 99%.
2.4 Non-ideal effects investigation
2.4.1 Light effect
The light effect caused the shift of sensing potential on pH detection during illumination is a serious problem for pH sensor application. In order to promote pH sensitivity, the sensing material with low light effect is more suitable for pH detection. Figure 2-6 shows the monitored C-V curves in dark-light-dark three steps of HfO2 EIS structure.
The light-induced sensing potential shift of HfO¬2 sensing membrane was calculated by dark C-V curves at first and third step during illumination. Figure 2-7 shows the C-V curves shift of HfO2 and WO3 sensing membrane before and after illumination. It obviously observed the light-induced sensing potential shift of HfO2 sensing membrane (11.2 mV) is larger than WO3 sensing membrane (5.4 mV) at the point of 0.6 Cmax. It indicated that the WO3 membrane is more suitable for applying in pH detection.
The measured light effect of HfxWyOz (sample III), HfxWyOz (sample III), and HfO2 (sample IV) are shown in Figure 2-8. A decreasing of light-induced sensing potential shift were observed with increasing the ratio of tungsten oxide incorporation. For HfxWyOz (sample III) sensing membrane, the light-induced sensing potential shift was 6.2 mV which was close to the value of tungsten oxide sensing membrane.
Table 2 lists the detailed pH-sensing properties, including sensitivity and linearity for pH ranging between 2 and 12 and light-induced sensing potential shift of WO3, HfxWyOz and HfO2 sensing membrane.
2.4.2 Hysteresis effect
To evaluate the hysteresis effect, the C-V measurements were performed in different buffer solutions in the following steps: pH 6.99-4-6.99-9.81-6.99. The hysteresis width was calculated as a potential difference for sensing responses of the first and the last step at pH 6.99. It shows obviously that the hysteresis width of HfO2 membrane is better than WO3 membrane.
The calculated hysteresis width of HfxWyOz sensing membranes with different composited conditions, WO3 (sample I), HfxWyOz (sample III), HfxWyOz (sample III), and HfO2 (sample IV) were shown in Figure 10. A decreasing of hysteresis width was observed with increasing the ratio of HfO2 incorporation. The hysteresis width of WO3 (sample I), HfxWyOz (sample III), HfxWyOz (sample III), and HfO2 (sample IV) sensing membranes are 17.63, 10.3, 4.74, and 2.34 mV in the following steps: pH 6.99-4-6.99-9.81-6.99., respectively.
Fig. 2-1 Schematic EIS structure with co-sputtering system
Fig. 2-2 C-V curves of HfO2 and WO3 sensing membrane in pH buffer solution from pH 2 to pH 12
Fig. 2-3 pH sensitivity of HfxWyOz sensing membrane with different powers of composited process, 0/50, 150/50, 150/30 and 150/0 of Hf/W (w/w)
Fig. 2-4 pK-responses of HfxWyOz sensing membranes with different powers of composited process, 0/50, 150/50, 150/30 and 150/0 of Hf/W (w/w).
Fig. 2-5 Calculated pK-sensitivity of HfxWyOz sensing membranes with different powers of composited process, 0/50, 150/50, 150/30 and 150/0 of Hf/W (w/w)
Fig. 2-6 Monitored C-V curves in dark box-illumination-dark box three steps of HfO2 EIS structure
Fig. 2-7 C-V curves shift of HfO2 and WO3 sensing membrane before and after illumination
Fig. 2-8 Light effect of HfxWyOz sensing membrane with different power of process composited, 0/50, 150/50, 150/30 and 150/0 of Hf/W (w/w)
Fig. 2-9 Calculated hysteresis of HfxWyOz sensing membranes with different power of process composited, 0/50, 150/50, 150/30 and 150/0 of Hf/W (w/w)
Table 1 Deposition rate and rf power for targets
Sensing
Membrane Sample
No. Hf power (w) W power
(w) Dep. Rate
(Å/min)
WO3 I 0 50 6.46
HfxWyOz II 150 50 16.9
HfxWyOz III 150 30 12.97
HfO2 IV 150 0 10
Table 2 pH-sensing properties for pH ranging between 2 and 12 and light effect of HfxWyOz sensing membrane
Sample
No. Sensitivity
(mV/pH) Linearity Light effect
(mV)
I 32.8 97% 5.4
II 28.8 97% 6.2
III 41.3 100% 6.8
IV 50.9 100% 11.2
Table 3 pK-sensing properties for pK ranging between 1 and 100 mM and hysteresis effect of HfxWyOz sensing membrane
Sample
No. Sensitivity
(mV/pK) Linearity Hysteresis
(mV)
I 21.49 99.55% 17.63
II 26.84 100% 10.3
III 18.68 99.67% 4.74
IV 7.07 98.40% 2.34
Table 4 Composed of HfxWyOz sensing membrane by XPS
Sample
No. Hf
(%) W
(%) O
(%)
I 0 9.47 90.53
II 14.71 2.25 83.04
III 13.14 3.82 83.04
IV 26.44 0 73.56
CHAPTER III
Urea biosensor based on pH-sensitive HfO2 - EIS structure
3.1 Introduction
3.1.1 Background of urea biosensors
Urea which is an end product of protein metabolism plays an important role in various body functions, such as kidney. The normal urea level in serum ranges from 2.5 to 7.5 mM, and the increases of urea concentration causes renal failure, urinary tract obstruction, dehydration, shock, and gastrointestinal bleeding. Furthermore, urea is one of the indicators of protein-feeding efficiency [32] as the principal component of non-protein nitrogen in cow milk. The improvement of feeding with appropriate urea testing can result in lower feed costs, greater milk-protein yield, reproductive performance increasing, and minimal nitrogen excretion into the environment [33-34]. Urea estimation is important during environmental monitoring, which can be responsible for reduction in pH of soil.
Although there are many methods for urea detection, such as gas chromatography [35], calorimetry [36], and fluorimetry [37], etc. However, the pre-treatment steps for sample preparation are complicated and not appropriate for on-site monitoring. To improve these problems, more researches were published in recent years. The enzyme, urease is the most commonly element of bio-sensing for a urea biosensor.
Urea biosensors with urease on sensing membrane were immobilized with many enzyme immobilized methods, such as physical adsorption [38], entrapment [39], covalent bonding [40], cross-linking [41]. The H+ [42-46] or NH4+ [47-51] ions, which are the products of urea hydrolysis could be detected by urea biosensor. The chemical response of urea hydrolysis was presented as follows:
NH2CONH2 + 3H2O 2NH4+ + HCO-3 + OH- 3-1
In this work, the pH-sensitivity characteristics of HfO2/SiO2 and Si3N4/SiO2 sensing membranes based on EIS structure were investigated. To set the process of urease immobilization by covalent binding and cross-linking methods, the pUrea-sensitivity of the conventional Si3N4/SiO2-EIS structure was studied firstly. Then, the same process of urease immobilization set up already was treated on the HfO2/SiO2-EIS structure. The pUrea-sensitivity, detecting range, and linearity of HfO2/SiO2-EIS structure were all measured to compare with the conventional Si3N4/SiO2-EIS structure.
3.1.2 pNH4 detection and pH detection of urea biosensors
NH4+ and H+ are two products of the urea hydrolysis, and the two ions always are used to detect urea concentration. For urea detection by NH4+ ion detection, chemically modified FETs (ChemFETs) were used. The ChemFETs with poly(hydroxyethyl methacrylate) (polyHEMA) hydrogel and Siloprene ion selective membrane containing noactine as ionophore used as the basic structure of EnFETs, which is good in sensitivity and durability.
For H+ ion detection, there have been many materials which are good in pH sensitivity used as different devices of sensor to detect urea, such as Si3N4 [52], TiO2 [53], and so on. The enzyme immobilization on inorganic is based on Schiff’s base formation which is between amino type groups on inorganic surface, amino groups on the enzyme and aldehyde groups.
3.2 Experimental
3.2.1 HfO2-EIS structure fabrication
For the processing of EIS structure, the p-type (100) silicon wafers with the resistivity of 8-12 Ω•cm were used as a starting substrate after RCA cleaning. Then, 500 Å thickness of silicon dioxide was grown in dry oxidation. After that, 150 Å HfO2 sensing membrane was deposited by automatic layer deposition (ALD) where the film grown at 200℃ for 150 cycles by tetrakis (ethylmethylamino) hafnium (TEMAH) as precursor, H2O vapor served as the oxygen source and Ar gas was supplied as the purge and carrier gas. Each of the cycle took 0.5 second for TEMAH precursor, 1 second for H2O vapor served, and 3 seconds for Ar gas. The thickness of HfO2 was estimated according to the deposition rate and checked by ellipsometer. Next, the 300 nm-thick Al film was evaporated on the backside of both wafers to form the ohmic contact. To define the sensing area of EIS for pH detection, a negative-photoresist (SU8-2005, MicroChem) was used. The radius of the sensing area is 1 mm. Then the EIS chip with the dimension of 1×1 cm2 was cut and assembled on the Cu line of the printed circuit board (PCB). Finally, a hand-made epoxy (silicon elastomer) was used to encapsulate the EIS structure and the Cu line. The commercial Ag/AgCl (S120C) electrode is used as the reference electrode.
3.2.2 Chemicals
Urease (E.C. 3.5.1.5, type IX, at activity 20 kU), glutaraldehyde (GA), 3-aminopropyl-triethoxysilane (APTS), and urea (U5378, 98+%). All of solutions were from Sigma, and diluted with DI water but except for urea and urease solution. Urea and urease solution were diluted with 5 mM phosphate buffer, containing 0.1 M NaCl and at pH 6.
3.2.3 Enzyme immobilization with covalent bonding and cross-linking
The covalent bonding steps were shown in Figure 3-1. At first, the HfO2-EIS sample immersed in 10% hydrogen peroxide for 24 hrs to generate hydroxy bond to react with 9% APTS solution at 40 ℃ for 4 hrs and generate amide bond. Then, 10% of GA which is a dual function was used to crosslink amine bond on urease and APTS. Final, enzyme was dripped on a sensing membrane and storage at 4℃ fridge overnight, then rinsed non-immobilized enzyme by phosphate buffer.
3.2.4 Measurement system
To extract the pH and urea sensing properties, the capacitance-voltage (C-V) curves for EIS structures were measured by using HP4284A high precision LCR meter. The ac signal frequency was chosen at 500 Hz for all EIS samples. The gate voltages were applied to silver/silver chloride reference electrode for EIS structures. To obtain the steady pH response, all EIS samples were immersed in the reversed osmosis (RO) water for 12 h before measurement. All measurement setup were carried out in a Faraday cage at room temperature to keep from light and noise interference. The responsive voltages for all EIS samples were calculated with 0.6 Cmax. The actual pH values of standard buffer solutions were measured by pH meter before and after measurement for pH sensitivity calculation.
For urea measurement, C-V curve was measured during one minute to observe the response time. Furthermore, between each measurement, enzyme-EIS samples were storaged in 4℃
3.2.5 Buffer solution preparation
To investigate the urea sensing properties, the 5 mM NaH2PO4/NaOH phosphate buffer solution with 0.1 M NaCl of the ion strength was prepared as buffer electrolyte which the pH value was kept at 6. The concentrations of urea are 0, 0.1, 0.5, 1, 3, 5, 10, 15 and 30 mM, respectively. The standard pH buffer solutions (Merk Inc.) from pH 2 to pH 12 and urea buffer solutions from 0 to 30 mM were all measured by the pH meter before measurement for buffer solution condition check and calibration. The change of urea concentration was adding the urea in PBS by micropipette.
3.3 pH detection of HfO2 EIS structure
Before enzyme immobilization, the characteristic of pH detection need to check. In Figure 3-2, the C-V curves measured from the pH 2 to pH 12 buffer solutions HfO2 membrane was obtained. HfO2 membrane fabricated in ALD exhibited high pH sensitivity which is about 57 mV/pH for pH ranging between 2 and 12. Furthermore, HfO2 sensing membranes have high linearity between pH 2 to the pH 12 buffer solutions. To the good conformity, the pH sensitivity of HfO2 sensing membrane deposited by ALD is higher than which was deposited by sputtering.
3.4 pUrea detection of Enzyme-HfO2 EIS structure
The change of urea concentration was adding the urea in PBS by micropipette. Figure 3-3 shows the HfO2/SiO2/Si EIS structure with covalent bonding method. The urea sensitivity is 112.64 mV/log Curea and the linearity is over 98.00% in the concentration range between 0.5 mM to 10 mM. Figure 3-4 shows the HfO2/SiO2/Si and Si3N4/SiO2/Si EIS structure with covalent bonding method, HfO2 sensing membrane has larger signal than Si3N4 sensing membrane.
However, most of enzyme was adsorptive on HfO2 sensing membrane which resulted in the sensitivity decreased quickly. In the next chapter, the relation between cross-linking and pH value for GA solution was discussed.
Fig. 3-2 pH-responses of HfO2-EIS structures
Fig. 3-3 pUrea-responses of enzyme-immobilized HfO2 EIS structures by covalent bonding method
Fig. 3-4 pUrea-responses of enzyme-immobilized HfO2 and Si3N4 EIS structures by covalent bonding method
CHAPTER IV
Characteristics of ammoniated-HfO2 sensing films on EIS for urea detection
4.1 Introduction
Due to the time-consuming of covalent bonding process, more methods were published to simplify the process by coating polymer membrane [54-56] or increasing the active area [57-58] to enhance the efficiency of bonding. In the way of cross-linking process, the reaction between amine bond and aldehyde on glutaldehyde (GA) plays an important role for pUrea-sensing. Therefore, creation of amine bond on sensing membrane surface by inorganic method seems a possible way to fabricate the urea sensor. In this work, the NH3 plasma treatment by plasma enhance chemical vapor deposition (PECVD) was firstly proposed to replace the covalent bonding process that for amine bond generation. The proposed schematic process flow for the NH3 plasma treated ALD-HfO2/SiO2-EIS fabrication was shown in Figure 4-1. The sensing properties including pUrea-sensitivity, detecting range, linearity were all studied and the amine bond on surface was examined by kit measurement. For optimizing the conditions for urease response, the reactions between the amine bond and the GA with different pH value were examined.
4.2 Experimental
For the processing of EIS structure, the p-type (100) silicon wafers with the resistivity of 8-12 Ω•cm were used as a starting substrate after RCA cleaning. Then, 500 Å thickness of silicon dioxide was grown in dry oxidation. After that, 150 Å HfO2 sensing membrane was deposited by automatic layer deposition (ALD) where the film grown at 200℃ for 150 cycles by tetrakis (ethylmethylamino) hafnium (TEMAH) as precursor, H2O vapor served as the oxygen source and Ar gas was supplied as the purge and carrier gas. Each of the cycle took 0.5 second for TEMAH precursor, 1 second for H2O vapor served, and 3 seconds for Ar gas. The thickness of HfO2 was estimated according to the deposition rate and checked by ellipsometer. After the HfO2 deposition, Ar and NH3 were flowed into the chamber, which is in the ratio of 25 sccm : 100 sccm. For two conditions of NH3 plasma treatment, one is the power in 200 watt for 3 minutes, the other one is the power in 100 watt for 6 minutes.
Next, the 300 nm-thick Al film was evaporated on the backside of both wafers to form the ohmic contact. To define the sensing area of EIS for pH detection, a negative-photoresist (SU8-2005, MicroChem) was used. The radius of the sensing area is 1 mm. Then the EIS chip with the dimension of 1×1 cm2 was cut and assembled on the Cu line of the printed circuit board (PCB). Finally, a hand-made epoxy (silicon elastomer) was used to encapsulate the EIS structure and the Cu line. The commercial Ag/AgCl (S120C) electrode is used as the reference electrode.
After ammoniated-HfO2 EIS fabrication, the pH and pUrea detection steps were the same in the chapter 3.
4.2.1 Enzyme immobilization-covalent bonding and cross-linking
After the ammoniated-HfO2 EIS fabrication, the process steps of covalent bonding for hydroxyl bond and amine bond generation were replaced. Therefore, there are only two steps need to complete, 2.5% of GA which is a dual function was used to crosslink amine bond which generated by ammonia plasma treatment and urease. Final, enzyme was dripped on a sensing membrane and storage at 4℃ fridge overnight, then rinsed non-immobilized enzyme by phosphate buffer.
Furthermore, the urea sensitivity and life time about 2.5% GA in different pH value, pH 6, 7, 8, 9 were compared.
4.2.2 Measurement system
For pH and pUrea detection, the measurement system was the same as the chapter 3. Furthermore, during the measurement of life time for once a day, sample immersed in the pH 6 phosphate buffer solution in a refrigerator.
4.3 pH and pUrea detection of ammoniated-HfO2 EIS structure
There is almost no difference between 3 minutes and 6 minutes plasma treatment on HfO2 sensing membranes for the pH sensitivity. In Figure 4-2, the pH sensitivity was about 56 mV/pH with high linearity for the two plasma treatment times. And compared with pure HfO2 sensing membrane, there is no obvious about the plasma damage for pH sensitivity.
For urea detection, HfO2 sensing membrane with 6 minutes plasma treatment is much better in urea sensitivity than with 3 minutes plasma treatment, the results are shown in Figure 4-3. HfO2-enzyme-EIS also comparative with HfO2 sensing membrane with plasma treatment, and the results are shown in Figure 4-4. HfO2-enzyme-EIS has bigger signal than HfO2 sensing membrane with plasma treatment, and the signal of 6 minutes plasma treatment is near HfO2-enzyme-EIS. Therefore, the plasma treatment can be used to replace some of traditional steps and improved the urea sensitivity by increasing plasma treatment. Table 2 was shown the comparison of urea biosensors for different enzyme immobilization, HfO2-enzyme-EIS has much larger signals than those references, but lifetime is the problem which is need to be improved. Therefore, the relation between lifetime and pH value of GA solution was discussed.
It can be observed in Figure 4-5 that during the enzyme immobilization process, pH 9 of GA has the worst stability only for one day, and the sensitivity decreased quickly, the reasons probable are the adsorptive enzyme by reticulation of glutaldehyde [59] and the method of storage but not covalent bonding. The sample with GA solution in pH 8 has better stability than pH 9 value of GA for 3 days lifetime. The sample with pH 7 of GA can be measured at 9 days, which better than the sample with pH 6 of GA process. During the pH value of GA process, pH 7 is the best condition for cross-linking.
4.4 Analysis
4.4.1 X-ray Photoelectron Spectroscopy and Spectrophotometer
In Figure 4-6, the amount of NH2 generated by 6 minutes plasma treatment is more than 3 minutes plasma treatment. For the results, the regulation of plasma treatment can affect to improve a quantity of amine bond the increase enzyme immobilized on the sensing membrane.
Table 1 was shown HfO2 sensing membrane through ammonia plasma was detected by Spectrophotometer. The enzyme-HfO2-EIS can be reused by 10% HNO3 solution to remove the enzyme which has losed activity. After HNO3 solution cleaned, there are still amine bond on the surface.
(a)
(b)
Fig. 4-2 pH-responses of ammoniated-HfO2 EIS by NH3 plasma treatment with (a) 3 min at 200W and (b) 6 min at 100W
(a)
(b)
Fig. 4-3 pUrea-responses of enzyme-immobilized ammoniated-HfO2 EIS by covalent binding method with NH3 plasma treatment for (a) 3 min at 200W and (b) 6 min at 100W
Fig. 4-4 pUrea-responses of enzyme-immobilized HfO2 EIS comparative with ammoniated-HfO2 EIS by covalent binding method with NH3 plasma treatment for 3 min at 200W and and 6 min at 100W
(a)
(b)
Fig. 4-5 Lifetime of enzyme-immobilized ammoniated-HfO2 EIS by covalent binding with different pH values of GA: (a) linearity (b) urea sensitivity
Fig. 4-6 X-ray Photoelectron Spectroscopy of ammoniated-HfO2 with NH3 plasma treatment for 3 min at 200W and 6 min at 100W
Table 5 NH2 detection by kit of Sectorphotometer
Structure
Sensing membrane
Abs.
umol/mg
EIS
HfO2+ NH3 (enzyme cleaned)
0.095
9.3
HfO2+ NH3
0.072
9.9
Table 6 Comparison of immobilization efficiency for immobilization of urease to different methods
CHAPTER V
Summary
The main purpose of this research is to develop a technique of composed material which can keep good characteristics, such as high pH-sensitivity and high pK-sensitivity but restrain the non-ideal effects, such as light effect, hysteresis, and drift of both sensing materials. In this study, the co-sputtering sensing membrane which is composed of HfO2 and WO3 films with different ratio were proposed to improve the sensing characteristics. The obtained conclusions were described as follows:
1. For high pH-sensitivity and low light effect on pH detection, the HfxWyOz sensing membrane deposited by the given Hf/W power of 150 W/30 W with good pH-sensitivity of 41.3 mV/pH and low light-induced sensing potential shift of 6.2 mV was chosen as the promise condition for pH-sensing application.
2. For high pK-sensitivity, high linearity in wide sensing range, and low pH-sensitivity on pK detection, the HfxWyOz sensing membrane deposited by the given Hf/W power of 150 W/50 W with good pK-sensitivity of 26.84 mV/pK, high linearity of 99.67% in the concentration range between 1 mM and 100 mM, and low sensitivity of 42.01 mV/pH was chosen as the promise condition for pK-sensing application.
The reason of HfxWyOz sensing membranes with good performance on potassium ion detection could be the variation of pHpzc value due to WO3 (lower pHpzc) incorporation.
In following chapters, the high pH-sensitivity of ALD-HfO2 sensing membrane was extended application for urea detection. Moreover, to replace the time-consuming covalent bonding process, the NH3 plasma treatment was successfully presented on ALD-HfO2 sensing membrane. The obtained conclusions were described as follows:
1. The pUrea-sensitivity of enzyme-HfO2 EIS is over 100 mV/logC urea at the first day, but the sensitivity decreased quickly after first day measurement. The possible reason is that enzyme was adsorptived on the surface not by cross-linking.
2. The NH3 plasma treatment can replace the covalent-binding enzyme-immobilized process for hydroxyl and amine bonds generation. The longer time for plasma ammonia treatment has better urea sensitivity.
3. The pH value is important for enzyme immobilization. For GA solution used to crosslink between the enzyme and the amine on sensing surface, pH 7 is the best condition to maintain the stability of urea detection in solutions with pH value of pH 6, pH 8, and pH 9.
|
|
[1] A. Errachid, J. Bausells, N. Zine, H. Jaffrezic, C, Martelet, N. Jaffrezic-Renault, M. Charbonnier,” Analytical features of K+-sensitive membranes obtained by implantation in silicon dioxide films”, Mat. Sci. Eng. C 21 (2002) 9-13.
[2] S. Khumpuang, M. Horade, K. Fujioka, S. Sugiyama, Portable blood extraction device integrated with bio-medical monitoring system, Proc. SPIE 6037 (2006) 60370J1-60370J8.
[3] Linkohr, S., Schwarz, S.U., Krischok, S., Lorenz, P., Nakamura, T., Polyakov, V., Cimalla, V., Nebel, C., Ambacher, O.,” A novel functionalization of AlGaN/GaN-ISFETs for DNA-sensors”, MRS. Vol. 1202 (2010) 35-40
[4] Lee, D., Cui, T., “Low-cost, transparent, and flexible single-walled carbon nanotube nanocomposite based ion-sensitive field-effect transistors for pH/glucose sensing”, Biosensors and Bioelectronics 25 (2010) 2259–2264
[5] Juffali, W., Georgiou, P., Toumazou, C., “ISFET based Urea:Creatinine translinear sensor”, Electronics Letters Vol. 46, Issue 11 (2010) 746-748
[6] A. Rudnitskaya, A. Ehlert, A. Legin, Yu. Vlasov, S. Büttgenbach, “Multisensor system on the basis of an array of non-specific chemical sensors and artificial neural networks for determination of inorganic pollutants in a model groundwater”, Talanta 55 (2001) 425-431.
[7] J. Artigas, C. Jiménez, C. Domínguez, S. Mínguez, A. Gonzalo, J. Alonso, “Development of a multiparametric analyser based on ISFET sensors applied to process control in the wine industry”, Sensor. Actuat. B-Chem 89 (2003) 199-204.
[8] J. Alonso, J. Artigas, C. Jiménez,” Analysis and identification of several apple varieties using ISFET sensors”, Talanta 59 (2003) 1245-1252.
[9] W. F. H. Abdullah, M. Othman, M. A. M. Ali and M. S. Islam, “Knowledge Representation of Ion-Sensitive Field-Effect Transistor Voltage Response for Potassium Ion Concentration Detection in Mixed Potassium/Ammonium Ion Solutions”, American Journal of Applied Sciences 7 (2010) 81-88
[10] N. Abramova, A. Ipatov, S. Levichev and A. Bratov, “Integrated multi-sensor chip with photocured polymer membranes containing copolymerised plasticizer for direct pH, potassium, sodium and chloride ions determination in blood serum ”, Talanta, Vol. 79, Issue 4, (2009) 984-989
[11] P. Bergveld,” Development of an ion-sensitive solid state device for neuro-physiological measurements”, IEEE Trans. Biomed. Eng. BME-17 (1970) 70-71.
[12] C. D. Fung, P. W. Cheung, and W. H. Ko, “ Electrolyte-insulator-semiconductor field-effect transistor”, IEDM Tech. Dig. (1980) 689-692
[13] M. Esashi and T. Matsuo, “Integrated micro multi ion sensor using field effect of semiconductor”, IEEE Trans, Biomed. Eng., vol. BME-25 (1978) 184-192
[14] S. D. Moss, C. C. Johnson, and J. Janata, “Hydrogen, calcium, and potassium ion-sensitive FET transducers: A prelimary report”, IEEE Trans. Biomed. Eng., vol. BME-25 (1978) 49-54
[15] C. E. Lue, J. C. Wang, D. G. Pijanowska, C. M. Yang, I S. Wang, H. C. Lee and C. S. Lai, “ Hysteresis effect on traps of Si3N4 sensing membranes for pH difference sensitivity”, Microelectronics Reliability, Vol. 50, Issue 5, (2010) 738-741
[16] J. C. Chou, Y. F. Wang, “Temperature effect of the pH-ISFET based on the sol-gel prepared SnO2 membrane”, Proc. SPIE, Vol. 4602 305 (2001)
[17] J. L. Chiang, J. C. Chou, Y. C. Chen, “Sensitivity and hysteresis properties of a-WO3, Ta2O5, and a-Si:H gate ion-sensitive field-effect transistors”, Opt. Eng., Vol. 41, 2032 (2002)
[18] C.-M. Yang, C.-S. Lai, T.-F. Lu, T.-C. Wang, D.G. Pijanowska, “Drift and hysteresis effects improved by RTA treatment on hafnium oxide in pH-sensitive applications”, J. Electrochem. Soc. 155 (11) (2008) J326-J330.
[19] C.-S. Lai, T.-F. Lu, C.-M. Yang, Y.C. Lin, D.G. Pijanowska, B. Jaroszewicz, “Body effect minimization using single layer structure for pH-ISFET applications”, Sensor. Actuat. B-Chem 143 (2010) 494-499.
[20] C.-S. Lai, C.-M. Yang, T.-F. Lu, “Thickness effects on pH response of HfO2 sensing dielectric improved by rapid thermal annealing”, Jpn. J. Appl. Phys. 45 (2006) 3807-3810.
[21] C.-S. Lai, C.-M. Yang, T.-F. Lu, “pH sensitivity improvement on 8 nm thick hafnium oxide by post deposition annealing”, Electrochem. Solid St. 9 (2006) G90-G92.
[22] Yu. Mourzina, Th. Mai, A. Poghossian, Yu. Ermolenko, T. Yoshinobu, Yu. Vlasov, H. Iwasaki, M.J. Schöning, “K+-selective field-effect sensors as transducers for bioelectronic applications”, Electrochim. Acta 48 (2003) 3333-3339.
[23] Z.M. Baccar, N. Jaffrezic-Renault, C. Martelet, H. Jaffrezic, G. Marest, A. Plantier, “K+-ISFET type microsensors fabricated by ion implantation”, Mater. Chem. Phys. 48 (1997) 56-59.
[24] E.A. Moschou, N.A. Chaniotakis, “Potassium selective CHEMFET based on an ion-partitioning membrane”, Anal. Chim. Acta 445 (2001) 183-190
[25] O. Sneh, Clark-Phelps, R. B., Londergan, A. R., J. Winkler, T. E. Seidel, Thin Solid films, Vol. 402 (2002) 248
[26] X. Liu, S. Ramanathan, A. Longdergan, A. Srivastava, “ALD of Hafnium Oxide Thin Films from Tetrakis-(ethylmethylamino)hafnium and Ozone”, E. Lee, T. E. Seidel, J. T. Barton, D. Pang, R. G. Gordon, J. Electrochem, Soc. 152 (2005) G213
[27] Suntola, T. Automic Layer Epitaxy. In Handbook of Crystal Growth, Vol. 3, Part B: Growth Mechanisms and Dynamics, In Handbook of Crystal Growth 3, Thin Films and Epitaxy, Part B: Growth Mechanisms and Dynamics; Hurle, D. T. J., Ed. ; Elsevier: Amsterdam, 1994; Chapter 14.
[28] C.S. Lai, C.M. Yang, and T.F. Lu, “Thickness effects on pH response of HfO2 sensing dielectric improved by rapid thermal annealing”, Jpn. J. Appl. Phys., vol. 45, no. 4B, (2006) 3807-3810
[29] C.S. Lai, C.M. Yang, and T.F. Lu, “pH sensitivity improvement on 8 nm thick hafnium oxide by post deposition annealing”, Electochem. Solid State Letters, vol. 9, (2006) G90-G92
[30] S. Yoshida, N. Hara, and K. Sugimoto, “Development of a wide range pH sensor based on electrolyte-insulator-semiconductor structure with corrosion-resistant Al2O3-Ta2O5 and Al2O3-ZrO2 double-oxide thin films”, J. Electrochem. Soc., vol. 151, (2004) H53-H58
[31] M. Kosmulski, “The pH-dependent surface charging and the points of zero charge”, J. Colloid. Interf. Sci., vol. 253, (2002) 77-87
[32] A. J. Taylor, P. Vadgama, “Analytical reviews in clinical biochemistry: the estimation of urea”, Ann. Clin. Biochem. 29 (1992) 245-264
[33] J. L. F. C. Lima, C. Delerue-Matos, M.C.V.F. Vaz, “Flow Injection System with Potentiometric Detection for the Determination of Urea Content in Milks”, J. Agric. Food Chem, 46 (1998) 1386-1389
[34] D. Lefier, Bull. Int, Dairy Fed. 315 (1996) 35-38
[35] C.J. Patton, S.R. Crouch, “Enzymatic determination of urea by calorimetrically methods”, Anal. Chem. 49 (1977) 464
[36] A. Ramsing, J. Ruzicka, E.H. Hensen, “A New Approach to Enzymatic Assay Based on Flow Injection Spectrophotometer with Acid-Based Indicators”, Anal. Chim. Acta 114 (1980) 165
[37] F. Roch-Ramel, “An enzymatic and fluorophotometric method for estimating urea concentrations in nanoliter specimens”, Anal. Biochem. 21 (1967) 372
[38] A. Poghossian and M. Thust, Sens. Mat., Volume B, Issue 4, (2001) 207-223
[39] M. H. Wu, C. D. Lee and T. M. Pan, “High dielectric constant PrYxOy sensing films electrolyte-insulator-semiconductor pH-sensor for the detection of urea”, Anal. Chem., (2009) 36-41.
[40] Y. K. and I. Y., “Properties and Morphology of Bioceramics/Poly(D,L-lactide) Composites Modified by In Situ Compatibilizing Extrusion”, Journal Applied Polymer Science, Vol. 102, Issue 5, (2006) 4886-4896
[41] P. R. S., and S. K. A., Lang. Vol. 23, Issue 13, (2007) 7398-7403
[42] Rajesh*, V. Bisht, W. Takashima, K. Kaneto, “ An amperometric urea biosensor based on covalent immobilization of urease onto an electrochemically prepared copolymer poly (N-3-aminopropyl pyrroleco-pyrrole) film”, Biomaterials 26 (2005) 3863-3690
[43] Lakard B, Herlem G, Lakard S, Antoniou A, Fahys B, “ Urea potentiometric biosensor based on modified electrodes with urease immobilized on polyethylenimine films”, Biosensors Bioelectron, 19 (2004) 641-647
[44] L. T. Yin, Y. T. Lin, Y. C. Leu and C. Y. Hu, “Enzyme immobilization on nitrocellulose film for pH-EGFET type biosensors” , Sensors and Actuators B: Chemical Vol. 148, Issue 1,(2010) 207-213
[45] T. F. Lu, J. C. Wang, C. S. Lai, C. M. Yang, M. H. Wu, C. P. Liu, R. S. Huang, Y. C. Fang , “A novel flash-ion-sensitive field-effect transistor (FISFET) with HfO2/Gd2O3(Gd) nano-crystal/SiO2 sensing membranes under super nernstian phenomenon for pH and urea detection”, IEDM, 5424289 (2009) 26.4.1-26.4.4
[46] M. Swati, R. Srivastava, “Polyelectrolyte-coated alginate microspheres for optical urea sensing”, IEEE NANO 5394690 (2009) 846-849
[47] N. H. Chou, J. C. Chou, T. P. Sun, S. K. Hsiung, “ Measurement and Comparison of Potentiometric Selectivity Coefficients of Urea Biosensors Based on Ammonium Ion-Selective Electrodes”, IEEE SENSORS JOURNAL. Vol. 5, 6 (2005)
[48] M. L. Hamlaoui, K. Reybier, M. Marrakchi, N. Jaffrezic-Renault, C. Martelet, R. Kherrat and A. Walcarius, “Development of a urea biosensor based on a polymeric membrane including zeolite”, Analytica Chimica Acta, Vol. 466, Issue 1,( 2002) 39-45
[49] J. K. Yang , K. S. Ha , H. S. Baek, S. S. Lee, M. L. Seo, “Amperometric Determination of Urea Using Enzyme-Modified Carbon Paste Electrode”, Bulletin of the Korean Chemical Society Vol. 25, Issue 10, (2004) 1499-1502
[50] C. Stamm, K. Seiler and W. Simon, “Enzymatic biosensor for urea based on an ammonium ion-selective bulk optode membrane”, Analytica Chimica Acta, Vol. 282, Issue 2, (1993), 229-237
[51] U.B. Trivedi, D. Lakshminarayana, I.L. Kothari, N.G. Patel, H.N. Kapse, K.K. Makhija, P.B. Patel and C.J. Panchal, “Potentiometric biosensor for urea determination in milk”, Sensors and Actuators B: Chemical, Vol. 140, Issue 1, (2009) 260-266
[52] D. G. Pijanowska and W. Torbicz, “pH-ISFET based urea biosensor”, Sensors and Actuators B: Chemical Vol. 44, Issues 1-3, (1997) 370-376
[53] X. Chen, Z. Yang and S. Si, “Potentiometric urea biosensor based on immobilization of urease onto molecularly imprinted TiO2 film”, JEC, Vol. 635, Issue 1 (2009) 1-6
[54] Rajesh*, V. Bisht, W. Takashima, K. Kaneto, “An amperometric urea biosensor based on covalent immobilization of urease onto an electrochemically prepared copolymer poly (N-3-aminopropyl pyrrole-co-pyrrole) film”, Biomaterials 26 (2005) 3683-3690
[55] W. Sant, M. L. Pourciel, J. Launay, T. Do Conto, A. Martinez, P. Temple-Boyer*, “Development of chemical field effect transistors for the detection of urea”, Sensors and Actuators B, 95 (2003) 309-314
[56] A. Maaref, H. Barhoumi, M. Rammah, C. Martelet, N. Jaffrezic-Renault, C. Mousty, S. Cosnier, “Comparative study between organic and inorganic entrapment matrices for urease biosensor development”, Sensors and Actuators B, 123 (2007) 671-679
[57] N. Sofikiti, N. Chaniotakis, J. Grandal, M. Utrera, M. A. Sanchez-Garcia, E. Calleja, E. Iliopoulos, and A. Georgakilas, N. Sofikiti1, N. Chaniotakis1, J. Grandal2, M. Utrera2, M. A. Sanchez-Garcia2, E. Calleja2, E. Iliopoulos3, and A. Georgakilas, “Direct immobilization of enzymes in GaN and InN nanocolumns: The urease case study”, Appl. Phys. Lett. 95, 113701 (2009) 3227888- 3227890
[58] A. Tiwari, S. Aryal, S. Pilla and S. Gong, “An amperometric urea biosensor based on covalently immobilized urease on an electrode made of hyperbranched polyester functionalized gold nanoparticles”, Talanta, Vol. 78, Issues 4-5, (2009) 1401-1407
[59] Roosevear, A., Kennedy, J.F., Cabral, J.M.S., 1987b. In: Immobilized Enzymes and Cells. Adam Hilger, Bristol, p. 95
[60] T. Godjevargova, A. Dimov, “Immobilization of urease onto membranes of modified acrylonitrile copolymer”, J. Membr. Sci. 135 (1997) 93–98.
[61] C. Marzadori, S. Miletti, C. Gessa, S. Ciurli, “Immobilization of jack bean urease on hydroxyapatite. A model for soil enzymes”, Soil Biol. Biochem. 30 (1998) 1485–1490.
[62] M. Gutierrez, S. Alegret,M.D. Valle, “Potentiometric bioelectronic tongue for the analysis of urea and alkaline ions in clinical samples”,Biosens. Bioelectron. 22 (2007) 2171–2178.
[63] Rajesh, V. Bisht, W. Takashima, K. Kaneto, “A novel thin film urea biosensor based on copolymer poly(N-3-aminopropylpyrrole-co-pyrrole) film”, Surf. Coat. Technol. 198 (2005)231–236.
[64] A.P. Girard-Egrod, R.M. Morelis, P.R. Coulet, “Bioactive nanostructure with glutamate dehydrogenase associated with LB films: protecting role of the enzyme molecules on the structural lipidic organization”, Thin Solid Films 292 (1997) 282–289.
|
| |