Light-Driven Ion Transport in Nanofluidic Devices: Photochemical, Photoelectric, and Photothermal Effects

Open AccessCCS ChemistryMINI REVIEW1 Jan 2022Light-Driven Ion Transport in Nanofluidic Devices: Photochemical, Photoelectric, and Photothermal Effects Kai Xiao and Oliver G. Schmidt Kai Xiao *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] Department of Biomedical Engineering, Southern University of Science and Technology (SUSTech), Shenzhen 518055 Center for Materials, Architectures and Integration of Nanomembranes (MAIN), Chemnitz University of Technology, Chemnitz 09126 Institute for Integrative Nanosciences, Leibniz IFW Dresden, Dresden 01069 and Oliver G. Schmidt *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] Center for Materials, Architectures and Integration of Nanomembranes (MAIN), Chemnitz University of Technology, Chemnitz 09126 Institute for Integrative Nanosciences, Leibniz IFW Dresden, Dresden 01069 https://doi.org/10.31635/ccschem.021.202101297 SectionsAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Light-driven ion transport in nanofluidic devices is a phenomenon where ions move unidirectionally by consuming optical energy, either from low concentration to high concentration or vice versa. The light-driven unidirectional ion transport offers intriguing application potential in desalination and ion separation, osmosis energy harvesting, and ionic machines benefiting from the remote noncontact light stimulus. Here, we review recent progress in nanofluidic-based light-driven ion transport systems and emphasize similarities and differences in the three underlying working principles based on photochemical, photoelectric, and photothermal effects. The current challenges and future developments of light-driven ion transport in nanofluidic devices are discussed. We believed that this article encourages further innovation in this exciting and emerging research field. Download figure Download PowerPoint Introduction Nanofluidics is a research field that explores the transport of fluids, gases, and ionic species at the nanometer scale.1–4 This enables the precise regulation of transported substances within confined spaces and is especially important for high-resolution sensing, disruptive technologies targeting the water-energy nexus, and macro-biomolecular analysis. Of particular interest is the study of ion transport in nanofluidic devices5,6 because it has much in common with that in biological protein nanopores, for example, ionic gating, ionic rectification, ionic Coulomb blockade, and ion pump properties.7–14 All these fundamental discoveries in nanofluidics are beneficial to elucidate the unique ion transport mechanism in biological protein nanopores, which exhibit elaborate nanostructures, to tune ion transmembrane transport. Furthermore, the control and understanding of ion transport in nanofluidic devices are important to many chemistries and chemical engineering processes, including water desalination, membrane filtration, and energy storage in batteries and supercapacitors.15–20 Light-driven ion transport is an interesting phenomenon occurring in nanofluidic devices that possess certain material attributes.8,21–23 In these systems, the consumption of solar energy moves ions from a low to a high concentration to establish a chemical potential (active transport) or amplify ionic flow from a high to a low concentration (passive transport). The active ion transport phenomenon is common in microorganisms or green plants that pump protons or ions across cell membranes, generating an osmotic and charge imbalance via solar energy consumption.24–26 Therefore, this phenomenon is also referred to as an artificial light-driven ion pump when ions flow against a concentration gradient driven by light illumination in nanofluidic devices. Initially, certain chemical compounds (artificial photosynthetic apparatuses or photochromic molecular switches) inserted into the lipid bilayer were adopted to mimic the function of proton pumps.27,28 With the development of nanotechnology, nanofluidic devices soon revealed their potential to control ion transport via light. In contrast to light-induced ion directional transport via ion-binding shuttle molecules,27,29 light-driven ion transport in nanofluidic devices operates based on breakage of the symmetric surface charge distribution or chemical potential under asymmetric illumination (Figure 1). These asymmetric factors eventually yield transmembrane forces to drive ion transport. Figure 1 | Light-driven ion transport in nanofluidic systems originates from three different mechanisms, photochemical, photoelectric, and photothermal effects, by creating asymmetric surface charge distribution or chemical potential across nanofluidic devices under unilateral illumination. Download figure Download PowerPoint Compared with other external stimuli,30–32 light-driven ion transport in nanofluidic devices attains a unique superiority that expands its applications. First, light can be rapidly turned on and off, allowing the resulting nanofluidic activities to be controlled with an exceptional temporal precision similar to biological channels. This property enables the fabrication of nanofluidic-based ionic sensors or machines, which have been dominated by electrons, such as ionic photodetectors.33 Second, photoresponsive nanofluidic devices can be controlled at specific locations because light can be precisely projected in space. For example, optogenetics functions via the introduction of a foreign light-responsive nanopore into a target cell, endowing it with a targeted and extremely fast (millisecond scale) control of precisely defined events in complex biological systems.34 Functionally, light-driven ion transport in nanofluidic devices holds a similar application potential. Third, light energy is abundant. It is possible to integrate light-driven ion transport nanofluidic systems with other membrane-based systems to realize large-scale industrial applications, for example, light-enhanced osmosis energy harvesting.23 These advantages equip light-driven ion transport nanofluidic systems with promising applications to conceive new types of ionic machines, separation and desalination membranes, and light-electron energy conversion systems. To date, three different mechanisms, including photochemical,35,36 photoelectrical,8,22 and photothermal effects,37,38 have been proposed to explain the light-driven ion transport phenomenon (Figure 1). In general, photochemical and photoelectrical effects will break the symmetric surface charge distribution, while the photothermal effect will result in the breakage of chemical potential along the nanofluidic device. The asymmetric distribution of either surface charge or chemical potential will drive ion movement to balance the asymmetry. Despite the apparent mechanisms, however, the light-driven ion transport phenomenon has many inconsistent interpretations because light illumination typically results in complex and cooperative effects based on different materials. These photoinduced effects sometimes coexist, making it difficult to distinguish the primary factor of light-driven ion transport. Consequently, observed signals are easily misunderstood. Elucidating the fundamental mechanisms underlying ion transport in light-responsive nanofluidic devices is therefore imperative for the development of light-driven ion transport. In this review, different light-driven ion transport mechanisms are briefly summarized and classified. Moreover, anomalous ion transport phenomena are denoted and examined. By comparing their performance, specific characterization approaches are also proposed to classify various light-driven ionic nanofluidic systems. Photochemical Effect Light-driven ion transport based on the photochemical (reaction) effect occurs in nanofluidic devices that intrinsically respond to light or are modified by photoresponsive molecules. Either a photoisomerization reaction or photoinduced electron transfer process occurring in the nanofluidic device alters its surface properties, for example, surface charge distribution, resulting in unidirectional ion transport. Nanofluidic devices that are sensitive to light and experience a chemical reaction under illumination are rare because photochemical reactions, in this case, result in their destruction. Exceptions exist when the nanofluidic device experiences a reversible photochemical reaction. Zhang et al.36 reported a light-driven ion transport system via a single polyethylene terephthalate (PET) conical nanochannel fabricated with the well-developed track-etching method. The polyester PET nanochannels contain abundant surface carboxyl groups after chemical etching, including benzoic acid and its derivatives (Figure 2a). The carboxyl groups remain in dissociation equilibrium with carboxyl dimers at the very beginning state. Thereafter, extra energy obtained from illumination drives the carboxyl dimers to dissociate into monomers, resulting in more negatively charged carboxylate groups. The enhanced electrostatic interaction between the cations traversing the nanochannel and charged groups (–COO−) occurring on the inner walls facilitates ion transport against a 1.25-fold concentration gradient. However, it should be noted that the nanofluidic device considered in this work is an asymmetric conical-shaped PET channel, in which a built-in internal electrical field is generated to drive ion transport.13,39 Here, illumination increases the surface charge density, resulting in an enhanced ion transport effect. Figure 2 | Photochemical reactions result in the change of surface charge density and ion transport in nanofluidic systems. (a) Photoinduced dissociation equilibrium and corresponding ion pump property. CBase side and CTip side represent ion concentrations on the base side and tip side, respectively. Reproduced with permission from ref 36. Copyright 2016 American Association for the Advancement of Science. (b) Photoinduced molecules' decomposition and the corresponding ionic current and voltage. Reproduced with permission from ref 40. Copyright 2019 Wiley-VCH. Download figure Download PowerPoint Another way to realize light-driven ion transport is to modify nanofluidic devices with photoresponsive molecules, which then experience photochemical reactions under illumination resulting in surface charge density redistribution and transmembrane potential establishment. This approach is not new and has been reported to drive ion transport through modified Nafion membranes21 or liquid membranes.28 The advantage of nanofluidic devices, which replace Nafion and liquid membranes here, is their stability. All these systems function based on the same principle, namely, the proton-coupled electron transfer process.29 Wang et al.40 reported a light-driven proton pump transport system through a Janus graphene oxide membrane (JGOM). JGOM comprises two different segments obtained by sequentially depositing graphene oxide (GO) multilayers doped with photobase (BOH) and photoacid (HA) molecules (Figure 2b). Upon light illumination, the HA molecule releases H+ and forms the conjugate base A−, while BOH produces free OH− and yields positively charged B+ on the other side of the membrane, resulting in asymmetric surface charge polarization on the HA-modified GO (HA-GO) and BOH-modified GO (BOH-GO) multilayers and a consequent intramembrane proton concentration gradient. The proton transport rate of JGOM is nearly four orders of magnitude larger than that through HA-modified Nafion membranes21 and several times lower than that through the spiropyran-doped liquid membrane,40 mainly resulting from the different internal resistances. In this work, JGOM pumps proton transport against concentration gradients several times higher based on calculation. Notably, the photochemical reaction only drives the transport of specific ions (e.g., protons) in nanofluidic devices by relying on lipophilic H+-releasing molecules. Beyond the aforementioned examples of light-driven unidirectional ion transport, it is surprising that other photoresponsive nanofluidic systems studies have reported ionic gating properties (switching between the opened and closed states) but have ignored light-driven unidirectional ion transport, despite similar modification methods and the same photoresponsive compounds including spiropyran and azobenzene.41–43 Based on the principle of asymmetric illumination→asymmetric surface charge distribution→transmembrane driving force generation, a net zero-volt ionic current should exist.8 One of the possible reasons for the missing signal is that the net ionic current resulting from the photochemical reaction is too low. Nevertheless, it is valuable to compare the ion transport properties under asymmetric and symmetric illumination conditions. If this hypothesis holds true, the scope of photochemical reaction-driven unidirectional ion transport may be remarkably expanded. Photoelectric Effect The photoelectric effect considered here refers to the excitation of electrons or other charge carriers to a higher energy state under light absorption, resulting in a voltage and current.44 More specifically, this refers to the light-induced separation of electrons and holes in semiconductor materials. Of course, we blur the distinction between the photoelectric effect and photovoltaic effect here. Photoelectric emission separates charges via ballistic conduction, and photovoltaic emission separates them through diffusion. In either case, the excited carriers may cause a chemical reaction, called the photoelectrochemical effect.45 Beyond that, the separated carriers may also drive charged ions to move (ion transport coupled to photoinduced carriers) through an electrolytic solution if the energy is not enough to drive a chemical reaction. This occurs when the photoinduced carriers do not have a sufficiently large bandgap to split water (1.23 V) or do not occur at appropriate positions relative to the redox potentials. To drive ion transport via the photoelectric effect, directional carrier movement is essential. In particular, asymmetric factors, including asymmetric nanofluidic structures, asymmetric light illumination, and asymmetric material compositions, are necessary. The first light-driven ion transport system based on the photoelectric effect was reported by Zhang et al.,46 who fabricated a self-organized TiO2 nanotubular array. Light irradiation from one side of the TiO2 nanotube resulted in a responsive current of 68 nA. The authors ascribed the ionic current response to light to the synergistic effect between the light-induced negative charges and asymmetric structure of the nanotube. To a certain extent, this is correct because photoinduced holes will likely move away from the n-type TiO2 nanotube surface. Simultaneously, the authors neglected the carrier concentration gradient along the nanotube, that is, the surface charge density gradient, under unilateral illumination, which may explain the zero-volt ionic current. Subsequently, Xiao et al.8 designed a carbon nitride nanotube (CNN) nanofluidic system for light-driven ion transport and provided a detailed mechanistic analysis. Their CNNs drive ions thermodynamically uphill against a 5000-fold concentration gradient under unilateral illumination by 380 mW/cm2 (Figure 3a). Moreover, a high light-driven current density of 2.4 μA/cm2 and a sustained open-circuit voltage of 550 mV are reliably generated. Like TiO2, carbon nitride is an n-type semiconductor material in which electrons are the primary carriers. The electrons separate from holes and move to the bulk carbon nitride or unilluminated side when illuminated from one side of the nanotube, resulting in a positively charged surface on the illuminated side. In contrast, the unilluminated side remains negatively charged. This is the origin of the asymmetric surface charge distribution across the nanotube. In other words, the voltage difference between the illuminated and unilluminated sides drives ion transport against the concentration gradient. Similar light-driven ion transport properties were also observed in MoS2 nanopores by Graf and Radenovic.47 Figure 3 | Light-driven ion transport by the photoelectric effect. (a) Photoinduced separation of electrons and holes in a single-phase semiconductor material and corresponding ion pump property. CHigh and CLow represent the high and low ion concentrations, respectively. Reproduced with permission from ref 8. Copyright 2019 Springer Nature. (b) Photoinduced separation of electrons and holes in semiconductor heterojunction and corresponding ionic current. Reproduced with permission from ref 23. Copyright 2021 Oxford Academic. Download figure Download PowerPoint Charge carrier recombination is one of the main factors causing a low performance of photocatalysis and photoelectrochemical cells.48 One effective way to mitigate this drawback is to fabricate semiconductor composites with different bandgaps. Multicomponent or multiphase heterojunctions may effectively reduce the recombination of carriers by capturing electrons and holes separately. Light-driven ion transport may also be affected or improved in the same way (Figure 3b). Xiao et al.23 reported a C3N4/TiO2 nanotube (CNTN) semiconductor heterojunction nanofluidic device that realizes a higher performance of light-driven directional ion transport than that of carbon nitride nanofluidics. Under the same light illumination, the light-induced ionic current was enhanced to 9 μA/cm2, which was ascribed to the more efficient photoinduced carrier separation. Many semiconductor materials are suitable for processing into nanofluidic devices to realize light-driven ion transport based on the following principles: suitable bandgap, good light absorption, excellent stability, and easy fabrication. Regarding a suitable bandgap, the material should exhibit the property of photoinduced separation of electrons and holes but should not produce enough energy for water splitting. It should be noted that the bandgap may be slightly modified by fabricating different nanostructures in certain cases because the structuring of absorbing semiconductor materials generates both positive and negative effects on its performance. For example, C3N4 with a bandgap of 2.7 eV is considered a photocatalyst for water splitting. However, the C3N4 nanotube membrane did not attain a photocatalytic performance in the absence of a sacrificial agent.8 Pure GO is regarded as a conductor material. Nevertheless, it sometimes has semiconductor properties benefiting from different fabrication methods or element doping, resulting in light-driven ion transport characteristics.22,49,50 By doping Pt particles, a metal–organic framework membrane can be adapted to produce a Schottky barrier photodiode and thereafter drive ion transport under illumination.51 Furthermore, double-(multi-)phase materials could provide more options to fabricate light-driven ion transport systems by constructing semiconductor heterojunction nanotube nanofluidic devices.23,50 Photothermal Effect Temperature differences can be converted into a voltage and current because the temperature gradient drives charge carrier diffusion from the hot to the cold side, known as the Seebeck effect.52 Similarly, the thermoelectric effect also occurs in nanofluidic-based ion transport systems.53 In contrast to electron transport-based thermoelectric systems, ions with a concurrent water flux are transported from cold to hot sides because the chemical potential of water linearly decreases with increasing temperature. Moreover, water molecules are prone to move from higher to lower chemical potentials. In nanofluidic systems, a temperature gradient across the nanofluidic device may easily be generated under light-driven heating, resulting in the phenomenon of light-driven ion transport via the photothermoelectric effect, denoted as the photothermal effect. The reported ion transport systems driven by the photothermal effect are mainly based on Ti3C2Tx MXene two-dimensional (2D) nanofluidic devices.54 Hong et al.37 reported that an array of nanoconfined Ti3C2Tx ion channels capture trans-nanochannel diffusion potentials under a light-driven axial temperature gradient. The thermoelectric response of MXene nanofluidic devices was calculated to reach up to 1.0 mV·K−1 under local sunlight exposure (Figure 4a). In this work, the magnitude and polarity of the generated open-circuit voltages are determined based on the thermostatic ionic flow from the unilluminated to illuminated sides. Moreover, the MXene nanofluidic device is very sensitive to the photothermal effect, where a very small temperature difference <;1 K results in an ionic voltage. This has been ascribed to the change in chemical potential induced by the photothermal effect. Under unilateral illumination, the electrolyte solution is heated due to local photothermal heating, resulting in a low chemical potential. Thereafter, thermo-osmotic ion transport occurs with a concurrent water flux from a higher to lower chemical potential, that is, from cold to hot sides. Figure 4 | Light-driven ion transport by photothermal effect induced temperature gradient. (a) Photoinduced chemical potential gradient and corresponding ionic voltage. Reproduced with permission from ref 37. Copyright 2020 ACS. (b) Photoinduced evaporation gradient and corresponding ionic current. Reproduced with permission from ref 56. Copyright 2020 Elsevier. Download figure Download PowerPoint Similarly, Liu et al.55 reported a Ti3C2Tx MXene-based ion transport system driven by the photothermal effect. However, they attributed the observed light-driven ion transport to the change in Gibbs free energy of the electrolyte in the illuminated regions. Regardless, the photothermal effect-induced temperature gradient should be the primary cause. Beyond that, a different mechanism was proposed by Lao et al.56 In their work, the measurement devices were slightly different because the 2D MXene nanofluidic devices were directly placed on top of the ionic solution (Figure 4b). They claimed that the light-driven ion transport observed in their system occurred due to the asymmetric evaporation of water through the MXene nanofluidic channels. This evaporation gradient drives water flow across the channel, converting thermal energy into the water kinetic energy, resulting in a streaming current because of the negatively charged MXene. Evaporation-induced electricity has also been observed in carbon materials, including graphene57,58 and carbon nanoparticle films.59 In all these studies, carefully designed control experiments verify the proposed mechanisms. For example, Lao et al.56 sealed their nanofluidic membrane with a thin layer of transparent plasma desorption mass spectrometry to verify that the current was driven by evaporation; the ionic current dropped from ∼12 μA to approximately 0.4 μA after sealing. However, previous findings have shown that evaporation is not the sole origin of the ionic current. More likely, the photothermal effect and intrinsic photoelectric effect of the MXene material operate in a collaborative way. It is evident that the photothermal effect is essential, and the temperature gradient resulting from illumination is the origin of the light-induced current and voltage. Challenges and Prospects Based on the examples mentioned above, it is easy to analyze the main features of light-driven ion transport in nanofluidic devices based on these three different mechanisms. First, asymmetric light illumination is necessary to create an asymmetric surface charge or chemical potential. However, photochemical and photoelectrical effects will lead to the surface charge redistribution, but the photothermal effect will result in the breakage of chemical potential. Despite clear theoretical models, the reported mechanisms are conflicting and require further examination to distinguish the origin of light-driven ion transport. Choosing light-responsive semiconductor nanofluidic devices as an example, light-driven ion transport could result from the photoelectrical effect originating from the direct excitation of carriers or the photothermal effect simply due to local heating caused by light absorption.60,61 Therefore, careful analysis is necessary to distinguish the origin of the ion pump function. One inconsistent example is the performance of GO membrane (GOM) based on photochemical reactions, which is much higher than that of GOM based on the photoelectric effect. In this case, a superimposed effect between the photochemical reaction and photoelectric effect should be the origin of the higher ionic current of GOM (Table 1). Of course, we must admit that a synergistic effect is inevitable in certain cases. However, regarding the study of the mechanism, one must determine the light-current origin. Thereafter, it is possible to enhance the performance by improving related parameters. Table 1 | The Performance Index of Different Nanofluidic Devices Is Categorized by Photochemical, Photoelectric, and Photothermal Effectsa Photochemical Reaction Photoelectric Effect Photothermal Effect Materials 1D PET single pore ( PET)362D-modified graphene oxide membrane ( GOM)42 1D Single MoS2 pore ( MoS2)492D Graphene oxide membrane ( nanotube ( nanotube ( 2D MXene ( MXene MXene membrane ( MXene current 2 2.4 μA/cm2 9 μA/cm2 MXene MXene voltage mV mV mV 550 mV MXene mV MXene gradient 1.25-fold 5000-fold MXene MXene to the to the to the to the to the MXene to the MXene to the material good stability, fast response good stability, response With material With material response calculated of MXene 1 and MXene 2 are To distinguish the different characterization methods have been proposed based on their Photochemical reactions in nanofluidic devices are by constructing nanofluidic materials or nanofluidic materials with light-responsive molecular including photochromic compounds or photoisomerization This process is by a change in surface chemical or the of specific which can then be via or other analysis The photoelectric effect occurs based on the photoinduced separation of electrons and of which are resulting in a Therefore, should be an effective to distinguish the photoelectric effect from the photochemical reaction and photothermal effect. In the of the photoelectric effect, the surface of the illuminated and unilluminated