Figure 1 shows a schematic of the structure and detection mechanism of a source-drain electrode coated with an insulation layer of submicron-order thicknessas obtained in this study. An ionic layer, like an electric double layer, forms at the surface of the solids and liquids [10]. When a solution containing metal ions with a concentration N is deposited onto the surface of an organic insulator with an adsorbent an electric double layer forms at the boundary surface(Figure 1(a)).Therefore, the insulator molecules become electrically polarized resulting in the formation of opposing charges on both the boundary surface(side (A) in Figure 1(a)) and the reverse side(side (B) in Figure 1(a)) of the insulator. This charge is detected by the source-drain electrode and is defined as the background voltage V_o.

Figure 1(b) shows the detection mechanism of the adsorbed chelate molecule. When a solution containing metal ions of the same concentration N is deposited onto the surface of an insulator with an adsorbed chelate molecule (Figure 1(b), side A) coordination reaction between the specific metal ion and the chelate molecule occurs resulting in a charge at the reverse side of the insulator (Figure 1(b),side B). In this case, the relationship between the source-drain electrode current and the sensor voltage follows Ohm's law [10]. The detection voltage is defined as V₁.

The adsorption ability of a chelate molecule on solid surface is obtained as follows: (1) Ion resonse = V 1 / V owhere V₁is voltage detected using an electrode chip adsorbed chelate molecule while V_o is the detected voltage of a normal electrode chip.

Figure 2 shows the fabrication steps for the electrode chip as a part of the preparation of the source-drain electrodes coated with an insulation layer and with chelate molecules. A Cr adhesion layer (0.1 μm thick) and a Au layer (1 μm thick) were coated successively onto a glass substrate (20mm×30mm in size and 1 mm thick) using the sputter method. A positive-type photoresist was first spin-coated onto the Au surface and then patterned by irradiating the photoresist through a Cr-coated soda-lime glass mask with UV light (Figure 2(1)).After irradiation the exposed regions of the resist were removed using a 25% tetramethylammonium hydroxide (TMAH) solution (Figure 2(2)). The underlying Auregions were then chemically etched using a KI-I₂ mixed solution (KI/I₂=1/1g, concentration: 17 wt%) (Figure 2(2)). Patterning of the Crlayer was also carried out using a ceric ammonium nitrate solution (concentration: 5 wt%)for chemical etching(Figure 2(2)). The remaining photoresist layer was removed by washing extensively in acetone(Figure 2(3a)). An overview image of an electrode chip is shown in Figure 2(3b).Five pairs of electrodes were included in the hexagonal eight array units and were arranged on a chip with an area of 15×28 mm. One array unit was composed of two source- (or positive terminal) and five drain- (or negative terminal) electrodes arranged within an area of 5×4 mm. The length, width and inter-electrode spacing of eachelectrode were fixed at 3mm, 150μm and 50μm, respectively. The electrodes were connected by pads arranged at the edges of the chip, which in turn are connected to an amplification circuit in the sensor signal analyzer. The fabrication and signal detection mechanism of this sensor signal analyzer is described in our previous report[10].

An insulator layer was formed on the source-drain electrode by first spin-coating a novolac resin solution with SiO₂ particles substituted with amino groups (Tosoh Co., 10μmφ, specific surface area: 200 m²/g) on the electrode and then drying at room temperature for 15 min(Figure 3(1a)). The insulator layer was then physically etched with a calcium carbonate particle solution using an ultrasonic cleaner. Chelate molecules dissolved in distilled water at a given concentration and volume were then deposited onto the insulator layer in the presence of an adsorbent (Figure 3(2a)). After adsorption treatment for 30min the electrode chip adsorbed chelate molecule was washed with distilled water to provide an electrode surface for evaluation.

Figure 3 shows an optical microscope image of the inter-electrode spacing before and after adsorption treatment. Here, copper phthalocyanine was used as a dye for staining. After the adsorption of the dye and washing with distillated water all the SiO₂ particles were stained a green color (Figure 3(2b)), whereas the color was transparent before the adsorption of the dye (Figure 3(1b)).

Figure 4 shows a developed wireless sensor-screening system for the detection of signals from the electrode chip. Before treatment of the chelate molecules adsorbed on the electrodes, the electrode chip was attached to a transmitter(overall size: 24.5×20×90.5mm, weight: 70g, Figure 4(a)). When a solution containing a specific ion was spotted on the array the converted digital signals from the electrodes were transmitted to a receiver(Figure 4(b)) located some 5m from the point of detection.

The developed wireless system in a package module “Miniship^TM” was incorporated into both the transmitter and the receiver (Figure 4(a,b)). The device, which has dimensions of 25×25×10 mm and a weight of 7g includes a PIC-type CPU. The power needed to drive this device (3.7V) is supplied by a Ni-H rechargeable battery fitted within the transmitter and/or attached via USB to a PC. The performance of the device has been reported previously [10,11].

The electrodes mounted on the electrode chip were connected to an R/V converter with an amplifier circuit. Each sensor signal is A/D converted by a CPU driven by a battery controller unit and a clock generator unit. The digital signal was transmitted by an RF transceiver (Figure 4(a)).A transmitted signal of 303.825MHz was received by the same device and was converted into a current signal by a serial/USB converter (Figure 4(b)). The current signal sent to the PC was then analyzed and processed using appropriate software.

The chelate molecules used for the adsorption experiment are given in Table 1.O,O′-bis(2-Aminoethyl)ethyleneglycol-N,N,N′,N′-tetraacetic acid (GEDTA) as a Ca²⁺chelator[CA(G)], O,O&prime-bis(2-aminophenyl)ethyleneglycol-N,N,N′,N′-tetraacetic acid, tetrapotassium salt, hydrate (BAPTA) as a Ca²⁺ chelator [CA(B)], 3-[3-(2,4-dimethylphenylcarbamoyl)-2-hydroxynaphthalene-1-yl-azo]-4-hydroxybenzenesulfonic acid, sodium salt (XB-1) as a Mg²⁺chelator (MG), 2-nitroso-5-[N-n-propyl-N-(3-sulfopropyl)amino]phenol (nitroso-PSAP) as an Fe²⁺ chelator [FE(N)], 4,7-diphenyl-1,10-phenanthrolinedisulfonic acid, disodium salt (BPDSA) as an Fe²⁺ chelator [FE(B)], 2,9-dimethyl-4,7-diphenyl-1,10-phenanthrolinedisulfonic acid, disodium salt (BCIDSA) as a Cu²⁺ chelator [CU(I)] and 5,10,15,20-tetrakis(N-methylpyridinium-4-yl)-21H,23H-porphine,tetrakis(p-toluenesulfonate) (TMPyP) as a Cu²⁺ chelator [CU(II)] were used for adsorption onto electrode chips. These commercial chemicals were provided by Dojindo Laboratories (Kumamoto, Japan).Solutions of CaCl₂, MgCl₂, FeCl₂ and CuCl₂ were used as guest molecules for adsorption onto the chelators. These commercial chemicals were provided by Wako Pure Chemical Industries, Ltd (Osaka, Japan).The combination of guest cations and chelators as host molecules for electrode detection are also summarized in Table 1.

Before an evaluation of the host-guest reaction on the electrode, a chelate dissolved in 200μL distilled water (concentration of 10mg/mL) was prepared and added drop wise to the surface of an insulator layer on the electrode chip. After adsorption treatment for 30min the electrode chip was washed with distilled water and used for an evaluation of the host-guest reaction.

Two kinds of cation solution were prepared for an evaluation of the adsorbed chelate molecules on the electrode chips: (a) only one type of cation dissolved in distilled water at three given concentrations of 1×10⁻²mol·L⁻¹, 1×10⁻⁴mol·L⁻¹ and 1×10⁻⁶mol·L⁻¹ (solution-free buffer) and (b) cations dissolved in a model buffer solution at three given concentrations of 1×10⁻²mol·L⁻¹, 1×10⁻⁴mol·L⁻¹ and 1×10⁻⁶mol·L⁻¹ (supplemental buffer solution). A model buffer solution was prepared using Na, K, Ca and Mg chloride (Na⁺ : K⁺: Ca²⁺ : Mg²⁺ = 95 : 3.3 : 1.7 : 1.0 mol for a total concentration of 1×10⁻²mol·L⁻¹) in distilled water (“DW”). The model buffer solution was added to the solution-free buffer of 1×10⁻²mol·L⁻¹ with a volume ratio of 1:1 (“standard solution”). The supplemental buffer solution was prepared by diluting the standard solution to 1/100 and 1/10,000. For Ca²⁺ or Mg²⁺ ion measurements a standard solution was prepared without the addition of CaCl₂ or MgCl₂, respectively.

Experiments were carried out as follows:

We have previously reported that the present sensor screening system rapidly detects voltage (within 100 ms) after the deposition of a sample solution and each electrode's profile was mostly stable [10]. Measurement of this sensor has been also performed. Therefore, the detected voltage taken between time intervals of 120 and 180s were averaged and an electrode voltage was obtained. Further more, the ion selectivity parameter, as defined by Equation(1), was also obtained.

Molecular orbital (MO) calculations for the chelate molecules were carried out with commercial software (MOPAC Vr.3, Fujitsu Co. Tokyo, Japan) using the PM3 method [12]. The energy level of the highest occupied MO (HOMO) and the lowest unoccupied MO (LUMO) were obtained by this calculation method. Furthermore, the HOMO and LUMO level of the model structure, as a surface of adsorbent, was also determined. Details are referred to in the discussion section.

Detection voltage details for the electrodes with the adsorbed chelate molecules are summarized in Table 2. Group1 contains four kinds of chelate molecules; CU(II), MG, FE(N) and FE(B), respectively. Figures 5 shows an example of the typical detection voltage of the host-guest interaction between the chelate molecules (Group1) adsorbed on the sensor electrodes and the cations. Figure 5(a) shows the detection of Cu²⁺ in pure water by CU(II) adsorbed on an electrode surface. The white plot of the solid line shows the relationship between the concentration of the Cu²⁺ ion and the voltage of the electrode without adsorption of the CU(II) chelate. The electrode voltage increased with an increase in the Cu²⁺ concentration giving values of 0.4 V at 1×10⁻⁶ mol·L⁻¹, 0.69 V at 1×10⁻⁴ mol·L⁻¹ and 1.24 V at 1×10⁻² mol·L⁻¹, respectively. The dotted line shows the logarithm is an approximate straight line. A coefficient of correlation R² value of 0.97 was obtained.

The black plot of the solid line shows the relationship between the concentration of Cu²⁺ ions and the voltage of the electrode with adsorbed CU(II) chelate. After the adsorption of the CU(II) chelate the detected electrode voltages increased to 0.7V and the voltage increased with an increase in Cu²⁺ concentration. The obtained values were 1.17 V at 1× 10⁻⁶ mol·L⁻¹, 1.41 V at 1× 10⁻⁴ mol·L⁻¹ and 1.96 V at 1× 10⁻² mol·L⁻¹, respectively. A coefficient of correlation value of 0.95 was obtained.

Figure 5(b) shows the detection of Cu²⁺ in the presence of a buffer solution by CU(II) adsorbed onto an electrode surface. The white plot of the solid line shows the relationship between the concentration of the Cu²⁺ ion contained in the buffer solution and the voltage of the electrode without the adsorption of the CU(II) chelate.

The electrode voltage increased with an increase in the Cu²⁺ concentration and values of 0.41 V at 1×10⁻⁶ mol·L⁻¹, 1.09 V at 1×10⁻⁴ mol·L⁻¹ and 1.55 V at 1×10⁻² mol·L⁻¹were obtained, respectively. The dotted line shows the logarithm is an approximate straight line. A coefficient of correlation R² value of 0.99 was obtained.

The black plot of the solid line shows the relationship between the concentration of the Cu²⁺ ions contained in a buffer solution and the voltage of the electrode with adsorbed CU(II) chelate. After the adsorption of the CU(II) chelate the electrode voltage increased with an increase in the Cu²⁺ concentration giving values of 1.29 V at 1×10⁻⁶ mol·L⁻¹, 1.45 V at 1×10⁻⁴ mol·L⁻¹ and 1.68 V at 1×10⁻² mol·L⁻¹, respectively. Electrode detection was hindered by the buffer solution and the voltage increased by 0.13V at 1×10⁻² mol·L⁻¹ of Cu²⁺ in a buffer solution after the CU(II) chelate was absorbed onto the electrode. A coefficient of correlation value of 0.99 was obtained.

Figure 5(c) shows an example of a typical ion response to a type I host-guest reaction. The detection of Cu²⁺by the CU(II) chelate that was adsorbed on an electrode surface is shown in Figure 5(c).The white plot of the solid line shows a relationship between the concentration of the Cu²⁺ ion in solution and the ion responsibility of the electrode adsorbed CU(II) chelate. The ion response decreased with an increase in Cu²⁺ concentration with values of 2.93 at 1×10⁻⁶ mol·L⁻¹, 2.04 at 1×10⁻⁴ mol·L⁻¹ and 1.58 at 1×10⁻² mol·L⁻¹, respectively. The black plot of the solid line shows the relationship between the concentration of the Cu²⁺ ion contained in a buffer solution and the ion response of the electrode adsorbed CU(II) chelate. Ion response decreased with an increase in theCu²⁺ concentration with values of 3.15 at 1×10⁻⁶ mol·L⁻¹, 1.33 at 1×10⁻⁴ mol·L⁻¹ and 1.08 at 1×10⁻² mol·L⁻¹, respectively. The ion response decreased to 35% at a concentration of more than 1×10⁻⁴ mol·L⁻¹ of Cu²⁺ in a buffer solution, while the ion response had no influence at 1×10⁻⁶ mol·L⁻¹ Cu²⁺ in a buffer solution and/or a buffer free solution.

The detection voltage details from the electrodes containing the adsorbed chelate moleculesare summarized in Table 3. Group 2 contains three kinds of chelate molecules: CU(I), CA(B) and CA(G), respectively. Figures 6 show an example of the typical detection voltage of the host-guest reaction between the chelate molecules (Group2) that are adsorbed on the electrode and the cations. Figure 6a shows the detection of Ca²⁺ in pure water by CA(G) adsorbed onto an electrode surface. The white plot of the solid line shows the relationship between the concentration of the Ca²⁺ ions and the voltage of the electrode without an adsorbed CA(G) chelate. The electrode voltage increased slightly with an increase in the Ca²⁺ concentration with values of 0.97 V at 1×10⁻⁶ mol·L⁻¹, 1.13 V at 1×10⁻⁴ mol·L⁻¹ and 1.18 V at 1×10⁻² mol·L⁻¹, respectively. The dotted line is a logarithm of an approximate straight line.A coefficient of correlation R² value of 0.92 was obtained.

The black plot of the solid line shows the relationship between the concentration of the Ca²⁺ ion and the voltage of the electrode adsorbed CA(G) chelate. After the adsorption of the CA(G) chelate, the detection voltage hardly changed and gave values of 1.09 V at 1×10⁻⁶ mol·L⁻¹, 1.17 V at 1×10⁻⁴ mol·L⁻¹ and 1.20 V at 1×10⁻² mol·L⁻¹, respectively. A coefficient of correlation value of 0.96 was obtained.

Figure 6(b) shows the detection of Ca²⁺ in the presence of a buffer solution by the CA(G) adsorbed onto an electrode surface. The white plot of the solid line shows the relationship between the concentration of the Ca²⁺ ions contained in a buffer solution and the voltage of the electrodes without the CA(G) chelate. The electrode voltage hardly changed and gave values of 1.11 V at 1×10⁻⁶ mol·L⁻¹, 1.27 V at 1×10⁻⁴ mol·L⁻¹ and 1.47 V at 1×10⁻² mol·L⁻¹, respectively.

The black plot of the solid line shows the relationship between the concentration of the Ca²⁺ ions contained in a buffer solution and the voltage of the electrode adsorbed CA(G) chelate. After the adsorption of the CA(G) chelate the electrode voltage increased slightly with an increase in the Ca²⁺ concentration and gave values of 1.15 V at 1×10⁻⁶ mol·L⁻¹, 1.18 V at 1×10⁻⁴ mol·L⁻¹ and 1.30 V at 1×10⁻² mol·L⁻¹, respectively. A coefficient of correlation value of 0.99 was obtained.

Figure 6(c) shows an example of the typical ion response of a type II host-guest reaction. The detection of Ca²⁺ by the CA(G) chelate that was adsorbed on an electrode surface is shown in Figure 6(c). In this case, the absorption of the CA(G) chelate made no difference.

To predict the interaction between the chelate molecules and the adsorbent the electronic orbitals of the chelate molecules and the surface model structures of the adsorbent was determined by MO calculations. The HOMO- and LUMO-values of each molecule are summarized in Table 4. Seven kinds of chelates, which were examined during the electrode evaluations, were subjected to calculations. The HOMO- and LUMO-values of the surface molecular structure are also summarized in Table 4.The SiO₂ surface model was assembled using five SiO₂clusters, which were minimized units. The clusters were further connected to an aminoalkyl group and a NH₂-R-(SiO₂)_n(R=–C₂H₄–NH–C₂H₄–, n=5) cluster was constructed. MO calculations for the assembled cluster model were also carried out and a calculated HOMO' value of−8.890eVwas obtained.

In accordance with frontier orbital theory [13],a chemical reaction between two molecules occurs easily when the difference between the HOMO- and LUMO-level is small. An amino group is present on the model surface structure as well as an NH₂-R-(SiO₂)_n cluster. The nitrogen atom in the amino group has a lone pair of electrons that can be donated to another molecular orbital. The lowest unoccupied MO (LUMO) of the chelate molecules can accept a lone-pair electron. Therefore, the difference between the HOMO'-level of the NH₂-R-(SiO₂)_n cluster and the LUMO-level of the chelate molecules were calculated. The difference is described as Δ(LUMO-HOMO') and is summarized in Table 4. Various values for Δ(LUMO-HOMO') were obtained. The CA(B) chelate gave the maximum value of 10.302 eV and the CU(II) chelate gave the lowest value of 5.472 eV, respectively.

Figure 7 shows the relationships between the actual measurement values of the ion response using an electrode chip and the calculated Δ(LUMO-HOMO') value. Figure 7(a) shows the ion response of all the chelate molecules when the measurement sample solution contained 10⁻²mol·L⁻¹ metal ions. The actual measurement value of the ion response decreased with an increase in the Δ(LUMO-HOMO') value; however, the 1st order approximation curve gave 0.62.

Figure 7(b) shows the ion response of each of the chelate molecules when the measurement sample solution contained 10⁻⁶mol·L⁻¹ metal ions. The actual measurement value of the ion response decreased with an increase in the Δ(LUMO-HOMO') value with a 1st order approximation curve of 0.77.