Keywords

1 Introduction

Creatinine is a waste product of Creatine Phosphate and Creatine in normal body muscle function. Creatinine (2-amino-1 methyl-5H-imidazol-4 one) is an amino acid compound first named by Liebig in 1847 [1]. About 2% of the creatine in body is converted into creatinine and 0.2–8 gm/l is excreted in the urine [2] and is a marker for kidney function [3]. Constant amount of creatinine is formed and released in Glomalular filtration, in time span of 24 h. For blood serum/plasma the normal range of creatinine is 34–140 μmolL−1 and it can go up to concentration over 1000 μmolL−1 in case of kidney disorder [4]. Therefore, development of a reliable, accurate and cost-effective process for creatinine detection is extremely important and noteworthy.

Creatinine determination in clinical laboratories is mostly based on Jaffe reaction which is a spectrophotometric detection. This method has low selectivity due to the interference from other biomolecules [5]. Amperometric biosensors have been developed for detection of creatinine [6]; however, these devices have low selectivity, stability and reproducibility [7, 8]. Among various concepts used for development of creatinine biosensors, the incorporation of enzyme with ion sensitive field effect transistor (ISFET) is one of the most captivating techniques. Using this concept, few creatinine sensitive ENFET devices (C-ENFET) have been developed in the recent past [9,10,11].

However, these devices have few disadvantages such as low sensitivity, high value of threshold voltage and small on-off current ratio. Using CNT in place of Silicon as channel material offer unique properties such as ballistic conduction, large surface area and high chemical stability [11]. Higher carrier mobility offered by CNT helps to improve sensitivity of the device as compared to silicon devices [12,13,14,15,16,17]. Moreover, compatibility of CNT with high-κ dielectric materials enhances device performance and also pave a way towards molecular electronics and devices miniaturization [14, 15]. For fabrication of such devices, electrochemical deposition (ECD) process has greater advantages as compared to the other microfabrication technologies presently used [18, 19].

For the fabrication of a CNT-BioFET different nanomaterials have been used. Zirconium dioxide (ZrO2) is used as the gate oxide layer as it is one of the most versatile ceramic materials with good physic-chemical properties. Its high strength, high melting point, low thermal conductivity, toughness, abrasion and corrosion resistance make it desirable for broad range of applications in technology. Given its benefits, ZrO2 is one of the most suitable candidates for gate dielectric material of in transistors [20,21,22].

The schematic figure and chemical bonding of ZrO2 and PEI doped COOH-MWCNT nanocomposite are given in Fig. 1(a) and (b) respectively.

Fig. 1.
figure 1

(a) Schematic figure and (b) Chemical bonding of ZrO2, PEI doped COOH-MWCNT and ZrO2.

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2 Experimental

2.1 Materials

Creatinine and Creatinine Deiminase enzyme were purchased from Sisco Research laboratory. COOH functionalized multi walled carbon nanotubes (MWCNTs) having carbon purity of ~99%, were purchased from Alibaba. Indium tin oxide (ITO) glass was purchased from NANOCS. Polyethylene imine (PEI) was purchased from Sigma-Aldrich. Tetraethyl Orthosilicate (TEOS) (28-28.8% SiO2) and Tetrapropylammonium Hydroxide 20% were purchased from Sisco Research laboratory (SRL). All the materials and chemicals were of analytical grade.

2.2 Fabrication

ITO glass with dimension ~10 mm × 10 mm × 1 mm was used as substrate for fabrication of ENFET device. The ITO glass was rinsed using a solution composed of water, ammonium hydroxide and hydrogen peroxide in ratio (5:2:2) and then washed thoroughly with distill water [23]. For electrochemical deposition three electrodes system was used, ITO glass (cathode) as working electrode, platinum wire as counter electrode and Ag/AgCl electrode as reference electrode. A solution has been made by hydrolyzing ZrCl4 in water (H2O) (10 mg of ZrCl4 in 10 ml of H2O) and ultrasonicated for 20 min [24]. On the substrate ITO glass, the resulting solution of ZrO2 was deposited by electrochemical deposition. The deposition was performed at room temperature, the resultant product after deposition was heated at 200 °C for 2 h.

Above ZrO2 layer, PEI-doped COOH-MWCNT has been deposited using chemical solution process. Two different types of solvent are used for the preparation of the CNT solution. First, 10 mg of CNT has been added to 10 ml PEI/methanol and ultrasonicated for 30 min [12]. Secondly, by changing the solvent with ethanol, CNT has been added to 10 ml PEI/ethanol and ultrasonicated for 30 min [25]. The resulting solutions have been deposited by spin coating and dried at room temperature. For the oxide layer, ZrO2 has been deposited above composite CNT layer using the same procedure.

A solution of silicalite was prepared in 5 mM PBS pH 7.4 using silicalite of 10% (v/v) and ultrasonicated for 10 min and then deposited over the gate surface by drop casting; after which, sample was heated for 15 min at 120 °C which will be used for the immobilization of enzyme. Tetraethyl Orthosilicate (TEOS) and tetrapropylammonium hydroxide (TPAOH) as a silica source and as a template respectively were used for silicalite preparation in molar ratio 1TPAOH:4TEOS:350H2O [26, 27]. Before the deposition of the enzyme, silicalite-ZrO2 layer was washed with distilled water to removed unbound silicalite from the ZrO2 surface.

3 Results and Discussions

3.1 Structures of the Deposited Layers

Figures 2 and 3 shows the XRD pattern of ZrO2 deposited on ITO glass and CNT composite. The two crystalline forms of ZrO2 i.e., monoclinic, tetrahedral structures can be seen on the deposited layers. In Fig. 2, the peaks on the XRD shows the presence of characteristic peaks of both monoclinic 2θ = (111), (022), (122), (002), (200) reflections [JCPDS No. 37-1484] [28] and tetrahedral crystalline 2θ = 30.2, 34.5, 50.2, and 60.2 equivalent to the (101), (110), (200), and (211) reflections [JCPDS No. 70-1769] [29] form in the film. The bottom ZrO2 layer acts as the insulating layer to avoid current leakage from transporting layer to ITO glass. In Fig. 3, ZrO2 characteristic peaks can be seen but the intensity of the peak is lower as the crystal is poorly crystalline in structure. Poor crystallization of ZrO2 deposited on CNT nanocomposite layers might be due to some effect of CNT layer beneath the deposited layer.

Fig. 2.
figure 2

XDR pattern of ZrO2 on ITO glass

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Fig. 3.
figure 3

XDR pattern of ZrO2 on CNT composite film.

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3.2 Morphology of the Deposited Layers

SEM analysis has been done after deposition of each layer for the study of surface morphology of the layers. SEM equipment (JEOL JSM 6390 LV, Singapore) was used for the investigation of the surface.

Figure 4(a) shows the SEM result of ZrO2 deposited on ITO glass, most of the deposited ZrO2 particles were in nanometer range.

Fig. 4.
figure 4

(a) SEM image of ZrO2 on ITO glass. (b) SEM image of ZrO2 deposited on CNT composite film.

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Figures 5(a) and (b) show the SEM image of PEI doped COOH-MWCNT in methanol and ethanol solvent respectively. The PEI doped COOH-MWCNT composite deposited on the ZrO2 surface is uniformly distributed in the second solution as compared to the first. The CNTs were dispersed homogeneously as the tubes of MWCNTs can be seen in the ethanol solvent much clearly than the methanol solvent, so the second sample is further used for ZrO2 deposition. Because of the amorphous loading of PEI coating on the surface of CNTs [30] and high loading of CNT in PEI, the CNTs are tightly bound to one another which make the CNTs increase its electrical conductivity. The CNT layer act as source, drain and channel of fabricated Bio-FET.

Fig. 5.
figure 5

(a) SEM image of PEI doped COOH-MWCNT using methanol as solvent and (b) SEM image of PEI doped COOH-MWCNT using ethanol as solvent

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Figure 4(b) shows surface morphology of ZrO2 deposited on the CNT composite layer. The particles are formed in an amalgam so the surface is non-uniform. The shape and size of the particles in ZrO2 deposited on CNT composite layer are different from the ZrO2 deposited on ITO glass.

Immobilization of Creatinine Deiminase can be done using adsorption technique on silicalite adsorbent membrane. Excluding the gate area, polydimethylsilaxane can be used to seal the device for the passivation of the layers to the electrolyte.

4 Conclusion

In this work, layers of CNT based Bio-FET has been deposited and physically characterized. High k-dielectric material ZrO2 and polymer nanocomposite of functionalized CNT were deposited using solution process. The crystal structure and morphology have been studied using XRD and SEM analysis. Results show good crystalline structure and surface morphology. Electrochemical deposition and spin coating process were explored for deposition of nanocomposite and oxide layers which is a new and cost effective methodology for fabrication of ENFET devices. In this study, efforts have been made to fabricate the transporting and sensitive layers of nano-structured CNT-BioFET for Creatinine detection that may play important role in bioelectronics applications in near future.