1 Introduction

The integration of new sensing materials with miniaturized is a fascinating fields of material science. It stimulates scientist for a wide range of areas such as molecular diagnostic, food industry, pollution environment, pharmaceutics [1,2,3,4,5].

In this framework, the development of new systems for the non-invasive biomolecules monitoring, such as glucose, aminoacids, protein etc. is receiving great attention due to its high impact to various diseases including diabetes and metabolic disorder (phenylketonuria) etc. [6,7,– 8]. In this context the glucose sensors devices are diffuse in different fields including food control, healthcare and industry. The traditional approaches to measure biomolecules levels are mainly based on enzymatic methods. In this scenario the non-enzymatic sensing based on the catalytic oxidation of biomolecules by nanomaterials represents the newest generation of glucose sensor technology [9,10,11]. Nickel is one of the most studied material since it allows the direct electro-oxidation of molecules by means of the active species NiOOH/Ni(OH)2 under alkaline medium. More in details, the sensing mechanism is based on the oxidation of analytes by the active NiOOH species, the produced Ni(OH)2 species are reconverted to NiOOH by applying a specific potential (~0.7 V). A nanosized layer of sensing species (NiOOH) on top of Ni(0) working electrode is formed by Cyclic Voltammetry experiments in NaOH 0.1 M [12,13,14]. In this contribution we present a silicon miniaturized three planar electrodes device integrating Ni zero-valence layer (thickness 10 nm) on working electrode. The device exhibited good response towards glucose detection on human blood and saliva samples. The sensing experiment indicates a strong dependence of the sensitivity from the pH values, in particular the sensitivity increases with the increasing of pH value. This biosensor pave the way to future development of versatile, easy-to-use, low-cost and portatile multiparametric chemical sensors.

2 Materials and Methods

2.1 Chemicals

Sodium phosphate (PBS powder, pH 7.4), NaOH pellets (purity 97%), glucose, Potassium Chloride, PBS tablet were purchased by Sigma-Aldrich and used as received.

2.2 Electrochemical Measurements

Cyclic voltammetry (CV) and chronoamperometry measurements were performed by a Verstat 4 (Princenton Applied Research), The CV experiments were executed with a scan rate 10 mV/s and a voltage range −0.1 V/+1.0 V. The nickel layer was activated by about 60 sweeps of cyclic voltammetry in NaOH 0.1 M. The chrono amperometry were performed at a fixed voltage of 0.48 V.

2.3 Saliva Sample and Pre-treatment

The saliva sample after collection was treated with a precipitation buffer (COPAN) to remove the protein interferences. In detail a volume of 200 µL of fresh saliva was mixed with same volume of precipitation-buffer, after 5 min a volume of 20 µL of the supernatant was analyzed with EC-device. For comparison chrono-amperometry measurements were performed for the precipitation-buffer.

3 Results and Discussion

The electrochemical device (EC-device) was manufactured using the VLSI technology on a 6″ silicon wafer substrate. It is composed by 4 electrochemical cells, each one containing three planar microelectrodes, a working electrode (WE) in nickel, a counter (CE) and a reference (RE) electrodes made in gold. The EC-device contains four reaction chambers each with a volume of 22 µl in volume (Fig. 1).

Fig. 1
figure 1

Details of measurement and device: a CV curves at NaOH 0.1 M, b three planar electrodes (from left to right: RE in gold, WE in Ni and CE in gold) and c EC-device layout

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The analytical performances for the EC-devices were evaluated measuring various amount of glucose on treated saliva sample. At this scope 10 µL of treated saliva was mixed with 10 µL of NaOH 0.1 M and chrono-amperometry measurement were recorded. After signal stabilization various glucose amount were added (from 20 to 250 µM) and the signal registered.

The Fig. 2 shows the experimental results of current intensity versus glucose amount, inset an example of chronoamperometric signal.

Fig. 2
figure 2

Current intensity versus glucose amount in solution. Inset chronoamperometric signal

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The data showed a limit of detection (estimated considering the current value corresponding to 3-times the current measured without glucose) of about 10 µM. These results encourage the development of miniaturized sensor integrating the Ni-device to be used in PoC format by not specialized end-users.

The future effort will be focused to improve the selectivity in complex biological matrices, the strategy will based on two approaches: implementation of algorithms in the analysis of sensing responses and integration of nanostructured Nickel species.

4 Conclusions

A miniaturized electrochemical device based on Nickel sensing material is here presented. The device is composed by three integrated metal microelectrodes with an active working electrode made of a Nickel. The sensing aptitude is based on the reversible conversion of Ni3+ (NiOOH) to Ni2+ (Ni(OH)2) interacting to redox active glucose molecules. The results herein presented proved that the system is able to measure the glucose amount on saliva after an easy precipitation of protein, with a detection limit of 10 μM. These data indicate that the proposed method is reliable and thanks to its miniaturized components such as the silicon microchip, it can be certainly considered a very promising for the development of a portable easy-to-use system for fast and effective monitoring of glucose on human specimen.