Self powered pH sensor using piezoelectric composite worm structures derived by ionotropic gelation approach

Highlights

• A novel piezoelectric composite wavy, linear worm structure has been developed for the first time.

• Energy harvesting and pH sensing functionality of composite worms has been reported.

• Length dependent output of CWPW structures and conversion of bio-mechanical to electrical energy has been studied.

• Developed self-powered pH sensor using CLW and CWPW structures.

• Worms are suitable for wearable, portable power sources without any storage components.

Abstract

Multifunctional biopolymer-piezoelectric composite worm structures (wavy and linear) derived by ionotropic gelation technique is fundamentally reported. Mass production of composite wavy pattern worms (CWPWs) enable high energy conversion from low frequency mechanical energy to electrical energy, tunable piezoelectricity by tailored length dependent CWPWs, weight ratio of piezoelectric nanoparticles. Interestingly we found that, the peak–peak voltage and current decreases around 87% and 71% for CWPW devices when the CWPWs length decreased to 56.4% (L = 1.95 to 0.85 cm) respectively. We also tested, the pH dependent conductivity of composite linear worm (CLW) for clinical, food monitoring applications. Next, we demonstrate the generated piezoelectric potential of CWPW device holds as a promising independent power source unit to drive the CLW sensor under different pH solutions. The proposed work is non-invasive, flexible with robustness in long-term effective usage, biocompatibility, and battery-less operation for self powered biosensor in theranostics, blood pH measurement.

Introduction

Alternative energy harvesting technologies like piezoelectric [1], [2], triboelectric [3], photovoltaic [4] and thermoelectric [5] devices can act as an independent power source using the environmental energies such as water motion [3], [6], human body movements [3], sun-light [2], [4], and air/wind flow [3], [7]. The output energy from these technologies is electrical energy, even though these are operated by various types of input energies such as sound, mechanical, wind, thermal and chemical energies. This electrical energy can be useful for broad range of applications from life-saving medical devices [7], [8], [9], [10], [11], [12] to portable devices [13], [14], [15], [16], [17], [18], [19]. On the contrary, other energies such as sound, thermal, chemical, mechanical and wind energies are useful energies, but these have inherent limitations for broad range applications. Meanwhile, the alternative piezoelectric harvesting technology generates electric power from μW/m² to W/m² by utilizing the sound, mechanical, wind and nuclear energies as an input energy source [20]. Moreover, the piezoelectric harvester with different micro/nanostructure materials has several merits such as low leakage current, biocompatible, highly stable from the interferences like temperature, pressure and humidity. But it has two ample drawbacks: one it suffers the development of highly efficient nano/micro structures by alone brittle piezoelectric ceramics with small scale production, high cost and time-consuming techniques like sol-gel, hydrothermal, physical vapor deposition (PVD) and chemical vapor deposition (CVD) techniques. Second, the fabrication of energy harvester and its performance was restricted by smaller dimension of single nanowire/belt [6], [7], [8], [9]. It directs to find alternative method [10], [11] for innovative structures with high energy conversion, sustainable at large input mechanical force and mass production with low cost.

However, the tuning of piezoelectric coefficient in nanomaterial is the key factor for achieving high energy conversion efficiency. This is possible in many ways such as the crystallographic orientation of lattice helps to increase an anisotropic properties of polycrystalline materials, creating internal stresses, doping foreign atoms into parent lattice and lowering dielectric constant of material respectively [2], [8], [9], [10], [20], [22]. Recent reports explore the reliable, large-scale nano/micro structures provides a low cost, high energy efficient conversion with compact working area and wearable property by the combination of piezoelectric nanoparticles (ZnO, BaTiO₃, PZT) along with the template polymers [23], [24], [25], [26]. This is due to the transfer of large mechanical force on active piezoelectric material via polymer matrix to tune the maximum piezoelectricity and provides the greater flexibility. Here, the selection of polymer matrix is a crucial factor to achieve the dual-functionality like energy harvesting and pH dependence with the same micro/nanostructure. Monitoring the pH value is a critical issue in many applications such as in-vivo biomedical, food monitoring, clinical and lubricant fields depend on pH variation processes [27]. Up to now TiO₂ [28], RuO₂ [29], SnO₂ [30], V₂O₅ [31] and ZnO [32], [33] metal oxides and surface plasmon resonance (SPR) based fiber optic sensors [34] were used for pH sensing/monitoring.

In order to overcome the above limitations, we developed the dual functional behavior of piezoelectric composite worm structures such as energy harvesting and sensing using ionotropic gelation (IG) approach [21]. The energy harvesting realized by innovative laterally aligned composite linear worm (CLW) and wavy pattern worms (CWPW) with multiple lengths consists of highly crystalline BaTiO₃ nanopartilces (BTO NPs) along with the 3D gel network of Ca-alginate (Ca-alg). The proposed single CWPW device has greater peak–peak V_OC ≈ 21.12 V and I_SC ≈ 2.53 μA (L = 1.95 cm, D = 0.055 cm) as compared to the single nanowire/micro belts. This is due to the crystallographic orientation of BTO lattice, lowering dielectric constant, internal stress of TiO₆ octahedron in BTO lattice and may be due to the acceptance of large mechanical force. This technique will solve the brittle nature and cracking of piezoelectric materials under the large mechanical force, mass production of worms with low cost and easy to fabricate multiple lengths based devices for greater energy conversion, flexibility. Next, we demonstrated the CLW as pH sensor under solutions with different pH values (alkaline value: 12–8) and self powered concept was realized by considering the CWPW device as power source unit. The pH solution dependent conductivity of CLW by metal (M)-semiconductor (S)-metal (M) interface is well matched with the electrical response of self-powered pH sensor. The proposed technique can pave way for developing the dual functional composites, which are more suitable in the fields of theranostics, vibration and portable devices with low cost, eco-friendly with high performances.