Use of a mobile phone for potentiostatic control with low cost paper-based microfluidic sensors
Introduction
The growth world-wide in cell phone usage since the late 1990s has been phenomenal. 80% of the globe now has wireless telecommunication coverage and world cell phone penetration is set to reach 100% within five years [1]. As a result of this volume their cost has remained surprisingly low, considering the remarkable expansion of the hardware and software capability of these devices. The power, ubiquity and connectivity of mobile phones creates opportunities to enhance health-care outcomes in the developing world via concepts such as telemedicine and e-health. For example, the use of mobile phones for remote monitoring of patients is becoming well established in dermatology and wound care [2], [3].
More recently, it has been realised that cell phones may be utilised for low-cost sensing applications in the developing world and remote regions [4], [5], [6]. The development of simple, inexpensive sensors for medical diagnostics and other applications is now an important emerging area in the field of chemical sensors [7], [8], [9], [10], [11], [12]. Two important criteria which need to be addressed in order to dramatically reduce the cost of point-of-care sensing are; firstly, the sensors must be mass-producible using cheap, readily available starting materials without the need for expensive fabrication facilities. Secondly, they should be readable without the aid of a dedicated scientific instrument [13]. The combination of cell phone technology with low-cost paper microfluidic sensors would fulfil these requirements; and in combination with telemedicine concepts, could facilitate a revolutionary approach to medical diagnostics in the developing world.
Several groups working in this area have explored the ability of cell phone cameras to act as detectors in low-cost sensing applications. The majority have focused on colorimetric detection [4], [14], [15], [16], [17], however the variability of ambient light or the necessity to introduce additional hardware for illumination has significant disadvantages. Fluorescence detection has been demonstrated [18], [19], [20], but this too requires external hardware for excitation, which increases cost and degrades simplicity. While chemiluminescence [21], [22], [23], [24] requires no excitation source, the difficulty of controlling the reaction timing complicates detection. Electrochemical detection [25], [26], [27], [28], [29], [30], [31], [32] also shows promise for detection in paper microfluidic sensors however the requirement for extra hardware to control the applied potential and measure the current again adds expense and complexity. This difficulty was overcome to some extent by Nie et al. [29] who used a cheap commercially available glucometer to interrogate their sensors; and elegantly by Crooks and coworker [33] who used an integral metal/air battery in the paper device to apply the potential and electrochromic read-out as an alternative to measuring current. We recently reported the use of electrochemiluminescence (ECL) detection in paper microfluidic sensors where a mobile phone camera served as the detector [34]. ECL, where a chemiluminescence reaction is initiated and controlled electrochemically, is an emerging detection technique [35], [36], [37], [38], [39], [40], [41], [42], [43] which is superior in many respects to photoluminescence because of the low background and electrochemical control aspects. This approach has significant advantages here; because ECL is performed in the dark, it is independent of ambient light and has the capacity for far greater sensitivity. Several other reports have since been published on ECL detection in paper-based devices [15], [16], [44], [45], [46], [47], [48], [49]. However, as with electrochemical detection, the problem of how to apply the necessary potential without recourse to external hardware remains a significant issue.
In this paper we describe how potentiostatic control can be achieved by exploiting another familiar feature of mobile phones; their ability to play music. By making an electrical connection to the sensor via the phone's audio socket and playing an appropriate sound file we show that a working electrode can be suitably polarised to initiate the electrochemical reaction leading to ECL emission. We show how both the electrochemical excitation and photonic detection processes can be software controlled using a custom written mobile phone application and we demonstrate proof-of-concept sensing of an important biological analyte. Because the generation, detection and analysis of the ECL signal is carried out solely using the mobile phone, the cost of the measurement is kept extremely low.
Section snippets
Materials and equipment
All solutions were made up in pH 7.5 phosphate buffer (0.1 M) unless otherwise stated. pH measurements were made using a MEP Instruments, Metrohm 827 pH Lab pH meter and a MEP Instruments Metrohm 6.0228.010 pH electrode. Adjustments to pH were made using 0.1 M HCl and 0.1 M NaOH. Tris(2,2′-bipyridyl)ruthenium(II) chloride hexahydrate (Ru(bpy)32+) (99% grade) was purchased from Strem Chemicals. l-Proline (99%), 2-(dibutylamino)ethanol (99%) (DBAE) and Potassium chloride (≥99.5%) were obtained from
Results and discussion
Fig. 1 shows the set-up used to demonstrate ECL generation and detection with a mobile phone. The sensors used consisted of a paper fluidic element loaded with the luminophore, Ru(bpy)32+, in face-to-face contact with a screen printed electrode as described previously [34]. However, the fabrication method used was different; briefly, instead of being patterned using a hydrophobic sizing agent, the filter paper was cut or punched to the desired shape after loading with the luminescent reagent an
Conclusions
The results presented herein, show that a mobile phone with suitable software installed can serve the basic functions of a potentiostat in controlling an applied potential to achieve electrolysis of redox-active molecules. If the molecules are ECL-active the resultant photonic signal can be monitored in place of the current. In combination with paper microfluidic sensors this creates a host of new possibilities for low-cost, instrument-free sensing with important implications for healthcare not
Acknowledgement
This work is supported by the Australian Research Council (ARC) through Discovery Project grant number DP1094179.
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