A Paper-Based Biological Solar Cell

Abstract

A merged system incorporating paperfluidics and papertronics has recently emerged as a simple, single-use, low-cost paradigm for disposable point-of-care (POC) diagnostic applications. Stand-alone and self-sustained paper-based systems are essential to providing effective and lifesaving treatments in resource-constrained environments. Therefore, a realistic and accessible power source is required for actual paper-based POC systems as their diagnostic performance and portability rely significantly on power availability. Among many paper-based batteries and energy storage devices, paper-based microbial fuel cells have attracted much attention because bacteria can harvest electricity from any type of organic matter that is readily available in those challenging regions. However, the promise of this technology has not been translated into practical power applications because of its short power duration, which is not enough to fully operate those systems for a relatively long period. In this work, we for the first time demonstrate a simple and long-lasting paper-based biological solar cell that uses photosynthetic bacteria as biocatalysts. The bacterial photosynthesis and respiration continuously and self-sustainably generate power by converting light energy into electricity. With a highly porous and conductive anode and an innovative solid-state cathode, the biological solar cell built upon the paper substrates generated the maximum current and power density of 65 µA/cm² and 10.7 µW/cm², respectively, which are considerably greater than those of conventional micro-sized biological solar cells. Furthermore, photosynthetic bacteria in a 3-D volumetric chamber made of a stack of papers provided stable and long-lasting electricity for more than 5 h, while electrical current from the heterotrophic culture on 2-D paper dramatically decreased within several minutes.

Introduction

For centuries, paper has been commonly used for biological and chemical analysis because of its biocompatible surface, hydrophilic porosity, and controllable optical properties.^1,2 However, it was not until paper-based microfluidics, known as paperfluidics, was introduced in 2007 that paper was reappraised as a game-changing material for next-generation point-of-care (POC) diagnostics.^2,3 The innovative aspect of paperfluidics lies in the use of a hydrophobic wax to create 3-D microfluidic pathways that distribute microliter volumes of analytes into multiple sensing regions of the paper to conduct multiple simultaneous bioassays. ⁴ The many unique advantages of paperfluidics, including extremely low-cost assembly, power-free fluidic control, and disposability by incineration, make it the best technique for single-use, disposable POC assays, particularly in limited-resource and remote regions.^5,6 Concurrent with advances in paperfluidics, remarkable efforts have been dedicated to paper-based electronics, known as papertronics, where paper is used as a substrate for electronic components because of its excellent dielectric constant.^7–9 The biodegradability of papertronics has attracted much attention as the future of green electronics, reducing the dramatic increase in electronic waste. Its easy and safe disposability make it ideal for such promising applications as security-related electronics and the Internet of Disposable Things (IoDT).^10,11 Merged systems incorporating paperfluidics and papertronics (named here as paper-fluidic-electro-systems) have the potential to yield exceptionally powerful functions and performances in POC diagnostics, reducing design complexity and enabling mass production in a cost-effective manner.^12,13 A novel electrofluidic fabrication method on paper for integrating electric pathways and microfluidic channels enabled the fluidic and electric interconnection and offered a number of new designs and capabilities for paper-fluidic-electro-systems.^14,15

To attain all-in-one fully sustainable and stand-alone paper-fluidic-electro-systems, their power sources must be built on paper for the system integration.^16–18 The fluidic control capability of paper allowed us to choose aqueous biological fuel cells as an energy supply. They are normally inexpensive, environment friendly, and easy to fabricate compared with solid-state power sources (e.g., lithium-ion batteries). ¹⁸ Recently, our group pioneered paper-based microbial fuel cells to realize a truly stand-alone and self-sustainable paper-fluidic-electro-system for POC testing.^{13,15,19–22} Through an integration of the microbial fuel cell into paper substrates and a preloading of lyophilized bacteria as a biocatalyst, a high-performance paper-based microbial fuel cell was created, allowing long device shelf life and on-demand activation with any organic matter, including wastewater and human saliva.^21,22 The device generated reliable power from microbial metabolism with one drop of the organic matter, delivering on-board energy to the next generation of paper-based POC platforms. This technique potentially enables a self-powered paper-based diagnostic test for anyone, anywhere, and anytime.^23–25

Although single-use, disposable POC tests require relatively small power consumption, the paper-based microbial fuel cells using heterotrophic bacteria have not been translated into practical applications because they fail to deliver power for a relatively long time. The heterotrophic bacteria required continuous replenishment of the bacterial nutrients for longer power generation, while the limited volume of the reservoir in a 2-D paper substrate did not contain enough nutrients. Even the latest advances in the paper-based microbial fuel cell technology generated power for only tens of minutes, hampering on-site, entirely self-sustained POC diagnostic testing. ²²

In this work, we created a paper-based biological solar cell to significantly improve power duration by using the sustainable energy production of photosynthetic bacteria ( Fig. 1 ). Biological solar cells are a type of microbial fuel cells that use photosynthetic microorganisms to convert light energy into electrical power, offering self-sustainable and self-maintainable features.^26,27 A very conductive and hydrophilic paper reservoir for bacterial electrogenicity, a solid-state cathode with reduced cathodic overpotential, and a transparent sealing layer for maximized solar energy capture were innovatively integrated into the paper-based biological solar cell. The cell produced a current density of 65 µA/cm² and a power density of 10.7 µW/cm², even higher than those of current micro-sized biological solar cells^28–30 and other paper-based microbial fuel cells.^13,22 Furthermore, a stack of wax-patterned papers formed a 3-D chamber to contain a sufficient amount of bacterial media, leading to relatively long power generation (~5 h).

Figure 1.

Schematic diagram of the paper-based biological solar cell and its cross section showing the individual layers.

Materials and Methods

Materials

Whatman Grade 3MM Chromatography paper (3030-861) and Ag₂O (AA11407-14) were purchased from VWR International, LLC (Philadelphia, PA). Cyanobacteria BG-11 freshwater solution, DMSO, glutaraldehyde solution, Whatman Nuclepore Track-Etched Membranes (111712), and phosphate-buffered saline (PBS) were obtained from Sigma-Aldrich (St. Louis, MO). Poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) (Clevios PH1000) was purchased from Heraeus (Vandalia, OH). Conductive graphite ink (cat. E34561000G) was obtained from Fisher Scientific Company, LLC (Wareham, MA).

Bacterial Inoculum

The photosynthetic bacteria Synechocystis sp. PCC 6803 were cultivated from −80 °C glycerol stock cultures by inoculating 15 mL of BG-11 medium with gentle shaking under a 12 h/12 h light cycle at 30 °C. The composition of BG-11 medium is as follows: 1.5 g of NaNO₃, 40 mg of K₂HPO₄, 75 mg of MgSO₄, 36 mg of CaCl₂, 1 mg of EDTA, and 6 mg of citric acid and ferric ammonium citrate per 1 L of distilled water. To compare the enhanced power duration of the biological solar cell, the heterotrophic bacteria Shewanella oneidensis MR1 was grown in 15 mL of L-broth medium with gentle shaking for 24 h at 37 °C. The L-broth medium includes 10 g of tryptone, 5 g of yeast extract, and 5 g of NaCl per liter. Bacterial growth was monitored by the optical density at 600 nm (OD₆₀₀). All bacterial samples were cultivated until their OD₆₀₀ reached 1.2.

Preparation of Paper-Based Biological Solar Cells

As shown in Figure 1 , the biological solar cell was constructed by sandwiching five functional layers: (1) transparent sealing tape (Clear Carton Sealing Tape, Duck), (2) 3-D paper chamber, (3) anodic paper, (4) proton exchange membrane (PEM; Whatman Nuclepore Track-Etched Membranes), and (5) cathodic layer. Except for the sealing tape and PEM, all were fabricated on Whatman Grade 3MM Chromatography paper. Hydrophobic barriers on the paper were formed by sequentially designing, printing, and heat-treating wax. The 3-D volumetric chamber was defined by stacking three wax-penetrated paper layers to hold an aqueous anolyte. Power duration could increase by adding more to a bigger chamber. The anodic layer was built by defining a hydrophilic region by a hydrophobic wax periphery. The layer was topped by printing a mixture of 1 wt% PEDOT:PSS and 10 wt% DMSO. That mixture made the paper reservoir conductive so electricity produced by the microbial metabolism can be effectively harvested ( Fig. 2a ).^15,22 Then, the graphite ink was screen-printed on the top part of that as the current collector. The cathodic layer was also prepared in the same way as the anodic layer except that a solid-state cathodic catalyst, Ag₂O, was introduced into the hydrophilic region. ¹⁵ A nanoporous filtration membrane (pore size 0.2 µm) was used as the PEM. Normally, size-selective membranes show superior proton transport capability and, thus, enhance power. ³¹ All layers with alignment cuts were carefully assembled and put together by spraying adhesive (3M Super77SprayAdhesive) ( Fig. 2b–d ). The anodic and cathodic surface areas were 0.503 and 1.960 cm², respectively.

Figure 2.

(a) Schematic illustrations of the fabrication process used to create the conductive and hydrophilic anode on paper. (b) Schematic diagrams of the biological solar cell, its operation, and its individual layers. Photo images of the device for testing (c) and size comparison (d).

Electrical Measurement Setup

A National Instruments (NI) data acquisition module (USB-6212) with a customized user interface was used for voltage measurements. Current and power generation were calculated with the connected external resistance and the measured voltage outputs via Ohm’s law. Current and power densities were normalized to the anodic surface area.

Bacterial Fixation and SEM Imaging

The anodic layers were washed gently with 0.1 M PBS and fixed using 4% glutaraldehyde solution overnight at 4 °C. Then, they were washed with PBS and dehydrated by 5 min serial transfers through 30%, 50%, 70%, 80%, 90%, 95%, and 100% ethanol. The samples were then placed in hexamethyldisilazane (HMDS) for 10 min followed by air-drying overnight. The fixed samples were examined using a FESEM (field emission SEM; Supra 55 VP, Zeiss).

Results and Discussion

Bacteria Loading and Operating Principle

A fluorescent lamp-controlled chamber provided continuous illumination throughout the experiments. After 100 µL of bacterial samples was introduced into the anodic chambers of two identical devices, we waited 30 min for bacterial acclimation and accumulation. The hydrophilic paper reservoir quickly wicked the bacterial solution through capillary action, allowing rapid adsorption of the bacteria onto the conductive paper fibers. As shown in Figure 3 , very densely packed photosynthetic bacteria were observed on the anodic paper, where individual bacterial cells were tightly attached to the paper fibers through extracellular protein fibers. ³² The anodic paper with pore sizes larger than 11 µm effectively worked as a scaffold to transport and trap the 1- to 3-µm-sized bacterial cells and form multilayer biofilms. The paper’s porosity created a large and accessible surface area enabling sufficient nutrient exchange to support internal bacterial growth and metabolism.^33,34 The photosynthetic bacteria continuously harvest electricity from microbial respiration and photosynthesis. During the respiration, the bacteria use bacterial nutrients stored in a BG-11 medium in the 3-D chamber, producing carbon dioxide and water and releasing electrons and protons. During the photosynthesis, the bacteria capture light energy to convert the carbon dioxide and water into oxygen and carbohydrates (to be used as bacterial nutrients), in which electrons and protons are consequently released again. The generated electrons travel through the external circuit, powering potential applications, while the produced protons move to the cathode through the nanoporous filter membrane, maintaining the electroneutrality of the biological solar cell (Figs. 1 and 2b). The electrons and protons at the cathode finally reduce Ag₂O to Ag, following the corresponding reaction:

$${\mathrm{Ag}}_{2}O+2H^{+}+2e^{-}\to 2\mathrm{Ag}+{\mathrm{H}}_{2}O$$

Compared with conventional microbial fuel cells that use chemicals or oxygen as the electron acceptor, the Ag₂O-based solid-sate cathode provides only a two-phase cathodic reaction of Ag₂O and water, thus eliminating low reactant collision probability or low aqueous oxygen solubility. ¹⁵ Therefore, the cathodic overpotential can be reduced, enhancing power performance. The transparent, gas-permeable sealing tape enclosed the chamber that holds the bacterial nutrients, maximized the light energy capture, and promoted gas exchange for bacterial photosynthetic and respiratory activities.

Figure 3.

SEM images of (a) the photosynthetic bacterial aggregates on the anodic paper and (b) the individual cells attached to cellulose paper fibers.

Power Generation and Duration

To quickly confirm the performance and reproducibility of the paper-based biological solar cells, they were first tested in an open-circuit configuration ( Figure 4a ). Then, the circuits were closed by connecting 100 and 10 kΩ external loads. The electrical performances were very similar, generating ~0.45 V of open-circuit voltage, ~0.37 V under 100 kΩ (i.e., 3.7 µA), and ~0.16 V under 10 kΩ (i.e., 17µA). However, the control without bacteria produced only negligible output voltage (~0.05 V), indicating that the solar cell device’s electrical outputs originated from bacterial photosynthetic and respiratory activities. Next, we examined the length of time that the solar cell device produced power. We compared the device that used a photosynthetic bacteria, Synechocystis sp. PCC 6803, with a device that used the well-known heterotrophic bacteria Shewanella oneidensis MR1 ( Figure 4b ). Both instantly generated high open-circuit voltages and gradually increased up to 0.47 V in 30 min. Then, we connected 100 kΩ resistors to the devices to allow the current to flow. A stable current of approximately 4 µA was generated for more than an hour from the photosynthetic bacteria, while the heterotrophic device exhibited a sudden current drop after operating only 30 min with its resistor. The photosynthetic bacteria continuously generated relatively higher current even at 4 h with a slight, gradual decrease, while the current generation from the heterotrophic device dropped to zero at 3 h. Although a 3-D volumetric heterotrophic chamber significantly enhanced the power duration from our previous 2-D paper-based devices,^15,22 the limited bacterial nutrients were rapidly depleted, generating unstable and considerably decreasing current output. On the contrary, the photosynthetic bacteria offered enhanced power duration with a relatively stable performance because photosynthesis reproduced nutrients the bacteria needed to produce power. Polarization curves and power outputs were measured and calculated twice, based on the maximum current value at a given resistance (470k, 250k, 162k, 100k, 71k, 47.5k, 32k, 22k, 15k, 10k, 2k, 1.5k, 0.45k, and 0.35k), the first after 1 h of operation and the second at 5 h ( Figure 4c and d ). At 1 h, the maximum power was 10.7 µW/cm² and the current density was 65.0 µA/cm², which are even greater than the power and current produced by existing micro-sized biological solar cells^28–30 and other paper-based microbial fuel cells.^13,22 These high power and current outputs are mainly because the porosity of the paper ensured a large anodic surface area for bacteria to attach and efficient mass transfer to and from the anodic paper. ³⁵ Furthermore, the strong affinity of bacterial cells toward the conductive paper fibers was promoted by capillary force and the engineered paper structure with biocompatible PEDOT:PSS polymer ( Fig. 3 ). ²² Finally, the reduced cathodic overpotential considerably increased power performance. ¹⁵ At 5 h, the electrical performance decreased to 0.44 µW/cm² and 15.0 µA/cm² mainly because of water evaporation through the gas-permeable sealing tape. With the addition of BG11 to the dried chamber, the performance went back to the original state (data not shown), demonstrating the self-sustainability of the biological solar cell. Further material discovery that meets transparent and gas-permeable requirements but minimizes water evaporation will significantly increase device practicability with enhanced sustainability.

Figure 4.

(a) Voltage outputs measured from the identical paper-based biological solar cells with the different resistors. (b) Voltage outputs measured from our device with photosynthetic and teterotrophic bacteria, respectively. Polarization curve and power output as a function of current density after (c) 1 and (d) 5 h operation.

Conclusion

In this work, we demonstrated an innovative paper-based biological solar cell to improve the power duration for potential POC diagnostic applications in resource-limited settings. Stand-alone and self-sustainable paper-fluidic-electro-systems will be essential to providing effective and lifesaving POC diagnostic devices even in those challenging fields. Our simple but powerful biological solar cell maximized bacterial electricity-producing capability in a well-controlled paper-based microchamber, featuring a transparent gas-permeable sealing layer, a solid-state Ag₂O cathode, and an engineered anodic reservoir. The device generated maximum power and current densities of 10.7 µW/cm² and 65.0 µA/cm², respectively, which are considerably higher than those of conventional biological solar cells. Furthermore, the power duration (~5 h) was significantly superior to that of the 2-D paper-based microbial fuel cells (several minutes). Our work will create a realistic and accessible power solution for interfaced, paper-based diagnostic devices that can revolutionarily improve the practical single-use applications in resource-limited settings.