Flexible nanostructured TiO2-based gas and UV sensors: a review

Flexible sensors have been attracting an ever-growing attention over the last years due to their outstanding characteristics, that include their lightweight and cost-effective characteristics, high stretchability, biocompatibility, and conformability. Moreover, the pursue of such devices has exponentially raised, with the IoT (Internet of Things) technology and the integration of several kinds of sensor devices that allow exchanging information on the internet, as well as remotely operating devices and collecting data. In fact, IoT is bringing sensor usage to a new level, where gas and ultraviolet (UV) sensors are largely integrated, guaranteeing the well-being and safety of people, with the immediate detection and response to changes in an environment. Gas and UV sensors based on titanium dioxide (TiO2) have been largely reported, where numerous efforts have been devoted to improving its sensing performance, especially when employing TiO2 at the nanoscale. TiO2 has the advantage of being chemical stable, non-toxic, inexpensive, and compatible with low-cost wet-chemical synthesis routes. This review outlines the current state of flexible gas and UV sensor technologies having TiO2 as the sensing layer and the impact of this nanostructured material on the field.


Introduction
In recent years, the research and development of different sensing technologies has evolved drastically, especially with the advent of IoT and the integration of different sensor devices for a common purpose, that is obtaining a connected and sustainable IoT system, and thus live on smart cities and comfortable/safe houses, while having automatized industries and competing businesses. In fact, IoT touches several sectors from healthcare to agriculture, passing through manufacture and retail, among others [1][2][3]. The complete concept is expected to give people the control of their own environment, saving money and avoiding waste.
IoT amplified the use of flexible devices, aiming to create and spread recyclable electronics, and thus reduce electronic waste. Moreover, such devices are generally cost-effective and compatible with large-area electronics, despite being adaptable to unlike surfaces, including curved ones, without losing their characteristics, and in some cases with a high stretchable character, and biocompatibility [4]. In fact, the biocompatibility of flexible devices allows the development of wearable electronics that can monitor human body information in real time [4,5]. Moreover, other distinct areas where flexible devices can be incorporated are packaging and flexible devices for portable electronics, such as supercapacitors, or in radio-frequency identification (RFIB) tags [6][7][8][9][10]. 1 3 on the semiconductor surface [52][53][54]. When target gases are adsorbed, electros are added or removed from this depletion layer, changing the sensor electrical conductivity. So, the conductivity variation results from a change in the charge carrier concentration. In the case of having oxidizing gases adsorbed on the n-type material surface, the gas species will act as acceptors, and the semiconductor will gain electrons from the adsorbed oxygen, which will increase the depletion region, and thus decrease its conductivity. For reducing gases, an opposite behaviour is observed, where the gas species act as donors, reducing the depletion region, and this will lead to the increase of conductivity [27,54]. Some examples of reducing gases are the H 2 , H 2 S, NH 3 , CO, volatile organic compounds (VOCs), while oxidative gases are NO, NO 2 , CO 2, and O 2 [29].
The UV sensing mechanism is related to the well-accepted gas sensing mechanism described above, as the semiconductor with the oxygen molecules adsorbed can capture free electrons and form a low conductivity depletion layer in the near-surface region. By the time that this semiconductor is exposed to a UV radiation source with energy higher than its band gap, electron-hole pairs will be photogenerated [hν → e − + h + ], in which the holes will migrate to the surface along the potential slope. The holes interact with the chemisorbed oxygen species ( O − 2 ), causing the oxygen to be desorbed from the semiconductor surface [h + + O 2 − (ads) → O 2(g) ]. This process leaves an excess of electrons in the conduction band, diminishes the depletion layer on the semiconductor surface, increasing its conductivity (Fig. 1). When UV radiation is turned off, oxygen re-adsorbs on the semiconductor surface, decreasing its conductivity. This hole-trapping mechanism through oxygen adsorption and desorption enhances the high density of trap states at the surface and thus enhances the semiconductor photo-response [27,55].
TiO 2 presents some bottlenecks to its application in sensors, one is that photogenerated electron-hole pairs are easy to recombine, leading to lower quantum yield [56]. In fact, metal oxide-based gas and UV sensors present limitations to their practical use, and in the case of TiO 2 gas sensors, the main drawbacks are their high operating temperatures, low selectivity, unstable signal, long response and recovery time, and lower stability when exposed to certain reducing or oxidizing gases [49]. While in the case of UV sensors, their performance is highly influenced by the surrounding environment, for example by presence of water vapor or by the atmosphere where the sensor is implemented [27,55].
To overcome these limitations, and as already mentioned, an alternative is the integration of a sensing layer based on TiO 2 nanostructured materials. As the sensing performance of the nanomaterials is directly related to their morphology, in the case of TiO 2, it has been reported several nanostructures for sensing applications, from nanoparticles to nanotubes, but also three-dimensional (3D) nanostructures such as nanoflowers that resulted in enhanced sensors [27,49]. Figure 2 shows different TiO 2 nanostructures used as sensing layers for detecting numerous targets, which includes TiO 2 nanoparticles, nanowires, nanotubes, nanospheres, and 3D structures [57][58][59][60][61][62]. Several techniques have been reported to produce these TiO 2 nanostructures including wet-chemical techniques [63,64], sputtering [65], electrodeposition [66], Fig. 1 A n-type semiconductor sensing mechanism (structural and band model) in the presence of target gases or UV irradiation. The barrier height is identified as (q.V S ). Reproduced from [52]  anodization process [60], hydrothermal and solvothermal synthesis [57,[67][68][69], microwave irradiation [40,44,[70][71][72], among others. Nevertheless, to maintain the low-cost character of the flexible devices, the production route of the TiO 2 nanostructures must also be considered. In fact, the integration of wet-chemical production routes in sensing applications has been growing with several advantages, including low energy consumption and low temperature processes, simplicity, and cost-efficiency without the requirement of sophisticated and expensive equipment. These techniques allow the precise control of thestructures at the nanoscale and their chemical composition, guarantee homogeneity, and are compatible with eco-friendly aqueous-based solutions [73].
When it comes to coupling TiO 2 with other materials to achieve a higher sensing performance, different approaches have been reported, including heterojunctions based on semiconductor/semiconductor, metal/semiconductor, organic/ semiconductor, and carbon-based material/semiconductor [29]. For the semiconductor/semiconductor heterojunction, it has been reported that the enhanced sensing properties is related to the heterojunction interface itself between TiO 2 and the other semiconductor. The work function (highest potential of valence band (VB)), electron affinity (lowest potential of CB), and band gap of the coupled semiconductors determine the electron/hole dynamics in the heterojunction. Moreover, this heterojunction may improve the electron mobility and enhance the electronic interaction, resulting in highest responses to target gases [74]. It is also attributed a synergetic effect between both semiconductor materials to increase the overall sensing performance, joining two materials that previously exhibited high sensitivity to a tested gas [29]. When it comes to metals, the catalytic activity of such nanoparticles has different effects on TiO 2 , including the increase of active surface area, reduce of electrical resistance and enhance of optical absorption, which will assist chemical adsorption of oxygen molecules and/or reaction of oxygen ions with target gases, and improve gas diffusion inside the heterostructure [29]. In the case of sensors' working temperature at room temperature, the coupling with metals are reported to increase charge carrier concentration and decrease activation energy promoting superior room temperature gas sensing [75]. a TiO 2 nanotubes for toluene sensing [57], b highly periodic 3D thin-shell TiO 2 nanostructures for NO 2 gas sensing [58], c TiO 2 3D hierarchical nanostructures for ethanol gas sensing [59], d TiO 2 tubular layers for NO 2 gas sensing [60], e image representing a flexible sensor device adapted from [34], f TiO 2 nanospheres for ethanol and acetone gas sensing [61], g TiO 2 nanoparticles for NO 2 gas sensing [62]. Reprinted with permission from "Large scale synthesis and gas-sensing properties of anatase TiO 2 three-dimensional hierarchical nanostructures, Langmuir 2010, 26, 12,841-12,848", copyright 2021 American Chemical Society [59], and from Elsevier [57,[60][61][62], MDPI [34], and John Wiley and Sons [58].
In terms of conductive polymers, they have been widely applied in room temperature gas sensors, lately. However, due to their relatively low conductivity and high affinity toward volatile organic compounds and water molecules, sensors with these materials still display low sensitivity, poor stability, and gas selectivity. To overcome these limitations, one alternative is coupling conducting polymers with inorganic nanomaterials, such as TiO 2 , and thus increase their sensing performance due to their synergistic effects [76]. When it comes to carbon-based materials, they usually exhibit a fast response to low concentration of a target element at room temperature owing to their low electronic noise, high surface area, and versatile surface chemistry, which can effectively enhance the overall TiO 2 sensing performance [76,77].
Several studies took advantage of the enhanced properties of TiO 2 nanostructured materials to the produce highquality and innovative flexible sensors capable of capturing several targets more efficiently and generating higher quality signals. Despite the plenty sensor devices where TiO 2 nanostructures can be incorporated as sensing layer, the present review will focus on flexible TiO 2 gas sensors and UV sensors/photodetectors. The studies compared in this review have in common the fact that the sensing devices used inexpensive, lightweight, adaptable, recyclable, and flexible substrates, with TiO 2 as the sensing layer.

Flexible TiO gas sensors
Nanostructured TiO 2 materials have been largely employed in gas sensors, and several studies described its sensing performance using flexible substrates, representing a potential alternative to the more expensive silicon technology used nowadays. It has been reported gas sensing devices using pure TiO 2 nanostructures/thin films as active sensing layers, however TiO 2 can also be coupled with other materials to enhance its sensing properties.

TiO 2 nanostructures/thin films for gas sensing
Several TiO 2 nanostructures have been tested as sensing layers for detecting different hazardous gases, and Kapton has been reported as an interesting substrate to be integrated in gas sensors. The advantages of such substrate rely on its conformability, high thermal stability, solvent resistance, cost-efficiency, enhanced electronic and mechanical properties, and high work temperature (+ 300 °C) [78]. A flexible gas sensor operating at low temperature was demonstrated in Ref. [78], where the TiO 2 membrane nanotubes prepared by anodization process were supported over Kapton substrate (Fig. 3). The sensor had its sensing performance tested with trimethylamine (TMA) gas within a concentration range of 40-400 ppm, with a response time of 20-25 s. In analogous studies, similar flexible sensors based on TiO 2 membrane nanotubes prepared by anodization and using Kapton as substrate were used for detecting d-butylamine within a concentration range of 8-20 ppm with a response time of 3 min [79] and for detecting hydrogen sulphide within a concentration range of 6-38 ppm [80], both at low operating temperatures.
Jang et al. [81] also reported TiO 2 nanotubes produced by anodization process on the titanium thin-films deposited on Kapton substrates (Fig. 4a). The process resulted in micrometer-long structures with open and regular tubular surfaces (Fig. 4b). These nanotubes were integrated on flexible chemoresistive sensors for detecting carbon monoxide (CO) and NH 3 gases. The gas responses were quite linear with respect to the different concentrations of test gases, reaching 68.3% for CO and 77.9% for NH 3 when exposed to 200 ppm at 350 ºC (Fig. 4c).
In Ref. [82], it has been shown a chemoresistive hydrogen gas sensor based on a TiO 2 thin film with simple interdigital platinum electrodes prepared on a Kapton polyimide foil. The sensor exhibited high response to hydrogen even at room temperature, which in dry conditions ranged from ~ 1.7 for 30 ppm H 2 up to ~ 104 for 10,000 ppm H 2 in synthetic air. At 150 °C, the response reached ~ 106 for 10,000 ppm H 2 . Humidity negatively affected the sensor performance particularly at room temperature. The sensor was bent 1000 times, without any significant damage of the device or decrease of sensitivity. Zhang et al. [83] reported the production of CO 2 sensors with TiO 2 nanowires deposited by electrodeposition on a flexible polyimide substrate and having printed silver electrodes. The printed CO 2 sensor revealed responses from 78 ppm to more than 1055 ppm at room temperature with a response time of 92 s and a recovery time of 25 s. The sensor also demonstrated to be slightly sensitive to humidity. The response is 2% for 1000 ppm CO 2 , while the response is 0.7% when the relative humidity changes from 48 to 99%. Ammonia, carbon monoxide, methane, hydrogen, and hydrogen sulfide were tested as interference gases, with a smaller response, confirming the greater selectivity of TiO 2 to the CO 2 gas. Ti wires functionalized with anatase TiO 2 nanotubes to produce one-electrode gas sensor have been reported in [84]. The sensor responses to acetone and alcohols have been demonstrated considering a detection limit below 1000 ppm.

TiO 2 -coupled nanostructures/thin films for gas sensing
As previously mentioned, pure TiO 2 has some limitations to its widely use in sensing applications, and in a way to surpass these drawbacks and enhance the sensing properties of the TiO 2 nanostructured sensors, several approaches have been reported, including coupling with other materials, like other semiconductor materials, conductive polymers, cellulose and metallic or non-metallic materials [49].
Wang et al. [85] reported the production and ethanol sensing performance of resistive gas sensors based on pure TiO 2 nanofibers and TiO 2 /Ag 0.35 V 2 O 5 branched nano-heterostructures (Fig. 5). The sensing properties of both sensors to 100 ppm ethanol vapor were determined, and when compared to the pure TiO 2 nanofibers sensor, the gas response of the TiO 2 /Ag 0.35 V 2 O 5 branched nanoheterostructures sensor has been reported to be dramatically enhanced by 9.1 times at 350 °C (R a /R g = 31.8 at 100 ppm) Moreover, it has been demonstrated that the TiO 2 /Ag 0.35 V 2 O 5 branched nanoheterostructures sensor exhibited a faster response/recovery time (7/8 s at 100 ppm), and a better selectivity characteristic towards ethanol. Fig. 3 Schematic of the flexible gas sensor based on TiO 2 membrane nanotubes prepared by anodization process. The SEM images of the nanostructures is presented together with the response of the sensor to TMA. Reproduced from [78] with permission of Elsevier In another study, TiO 2 nanoparticles on 2D-TiC (titanium carbide) nanosheets were deposited on PET substrate by spin-coating, and a flexible gas sensor was produced with the deposition of gold electrodes on the patterned substrate. This sensor revealed an enhance response and selectivity towards ethanol gas with a limit of detection up to 10 ppb with low noise to signal ratio [86].
Vidis et al. [87] reported a flexible hydrogen gas sensor with a capacitor-like Pt/TiO 2 /Pt electrode arrangement fabricated on a polyimide foil. The sensor had its hydrogen gas sensing properties and bending endurance evaluated, exhibiting high response, 100,000 to 10,000 ppm H 2 at 150 °C with minor decline at elevated humidity. The lowest detected concentration was 3 ppm at 150 °C and 300 ppm at room temperature in dry conditions. Another flexible hydrogen sensor has been reported in [88], where palladium and TiO 2 nanofilms were deposited on PDMS substrates. The influence of the TiO 2 film thicknesses were investigated and the sensor with 6 nm demonstrated the best performance with a response/ recovery time of 1.6 s/6 s at 0.2% (2000 ppm) hydrogen concentration in dry air at room temperature.
Cellulose has also been integrated in gas sensors. Tong et al. [89] demonstrated wearable gas sensors based on TiO 2 /cellulose nanocrystals (CNCs) composite films. The gas sensor, based on 6% TiO 2 /CNCs composite film, reached the highest response value (1.34-7.12) to NH 3 within the concentration range of 435-8750 ppm. The response and recovery times of this condition to 435 ppm NH 3 were 20 s and 40 s, respectively, demonstrating strong selectivity and stability to NH 3 gas at room temperature. Additionally, it has been shown that the response of the TiO 2 / CNCs gas sensor to 1750 ppm NH 3 was improved by 1.35 times under UV radiation. In Ref. [90], a cellulose-titanium c The gas sensing properties of the flexible gas sensors under a working temperature of 350 ºC for CO and NH 3 is also presented. Reproduced from [81] with permission of Elsevier dioxide-multiwall carbon nanotube nanocomposite was tested as NH 3 gas sensor at room temperature. The electrodes were directly patterned on the nanocomposite film. This gas sensor showed a potential for monitoring traces of NH 3 (50-500 ppm) with a good sensitivity and repeatability. Pang et al. [91] reported cellulose/polyaniline (PANI) and cellulose/TiO 2 /PANI composite nanofibers for detecting ammonia gas at room temperature. The response values of the composite nanofibers with the ammonia concentration of 10, 30, 50, 100, 150, 200 and 250 ppm were 0.584, 1.442, 2.334, 3.570, 4.801, 5.719 and 6.335, respectively. Moreover, it has been shown that the response value to 250 ppm ammonia is expressively higher than those response values to other gases, i.e. acetone, ethanol and methanol.
In another approach, but also integrating PANI in the sensor, NH 3 gas sensors based on PANI-coated TiO 2 -silicon dioxide (SiO 2 ) or PANI-copper oxide (CuO)-TiO 2 -SiO 2 composite nanofibers were fabricated by electrospinning, originating flexible membranes [92]. The copper oxide-based sensor revealed much higher response values to ammonia gas, reaching 1.46-400 ppb and 45.67-100 ppm ammonia at room temperature. A different PANI-based nanocomposite has been reported in [93]. In this study, a gas sensor array containing a sensing layer based on a nanocomposite film of 4 different elements, i.e. PANI, PANI/carbon nanotubes, PANI/tin oxide (SnO 2 ) and PANI/TiO 2 , and deposited on a flexible substrate, has been described. Standard technology for printed circuit board was used for the fabrication of the sensor. The sensor array was investigated for the detection of NH 3 , CO 2 , NO 2 , O 2 , acetone, toluene and relative humidity at room temperature. The responses to ammonia for the PANI nanocomposite films were reversible and repeatable [93]. Khasim et al. [94] reported a highly flexible and stretchable composite sensors based on poly (3,4-ethylenedioxythiophene: poly(styrene sulfonic acid)) (p-PEDOT-PSS) and TiO 2 treated with diethylene Glycol (DEG) using a simple bar coating. These ultra-sensitive nanocomposite films were employed as resistive sensors for the detection of nitric oxide (1-250 ppm) at room temperature, exhibiting a sensitivity of 96% (@ 250 ppm), and response/recovery times of 158 s and 54 s, respectively. It has been shown that the sensor has enhanced sensitivity retention under mechanical deformations such as bending and stretching. The effect on gas sensing response varying the humidity range 10-90% RH was reduced, indicating that the prepared sensor composite is stable for extreme humidity conditions.
A sensor based on polyoxometalates (POMs)-loaded TiO 2 -nanocrystal arrays using a polyimide substrate has been reported in [95]. Its sensing performance has been estimated by the contactless detection of triacetone triperoxide at room temperature, in which its response under 365, 450 and 550 nm illumination is 81%, 42%, and 37%, respectively, being able to detect less than 50 ppb. UV and visible light exposure are a useful way to improve gas-sensing properties and reduce the working temperature. The flexibility and stability of the flexible sensor film was demonstrated, and it has been shown that the sensing response was significantly depended on the illumination wavelength, however not affected by ambient air below 60% relative humidity.
Tang et al. [96] demonstrated a chemical sensor for detecting formaldehyde, with a polypyrrole-based molecularlyimprinted polymer (PPy-based MIP) employed as the sensing recognition layer, and synthesized on a high surface-tovolume ratio TiO 2 nanotube array. A Ti sheet was used for the anodization process and as substrate. It has been shown a response of 13% for 1 ppm formaldehyde at room temperature, with good selectivity towards formaldehyde compared to acetone, acetaldehyde, acetic acid, and ethanol. This sensor also demonstrated stability of more than one year and rather good immunity to humidity.

TiO 2 multi-sensing sensing devices
Flexible devices for simultaneously gas and UV sensing have also been reported [97][98][99]. Dubourg and Radovic [99] developed a new strategy for producing multi-sensing devices for UV detection and ethanol sensing, which was based on printed TiO 2 nanoparticles with laser-tunable properties. The approach developed results from the association of two different techniques: screen-printing and laser treatment process (Fig. 6a). The first technique is for the patterning, and the second was adapted for the post treatment of the active layers on flexible substrates, simultaneously and selectively sintering the TiO 2 patterned films in a single step. The sensor used anatase TiO 2 nanoparticles, screen-printed silver electrodes and PET substrates. A complete study has been carried out regarding the effect of laser treatment on the sensing performance, but also the effect of laser fluence. In the UV sensing measurements, the untreated sample reached 1.36 nA at 5 V, while the sample treated with 0.21 J cm −2 has three order of magnitude higher current (341 μA at 5 V). The response/recovery times were estimated for the laser treated sensor (0.19 J cm −2 ), revealing that the photoresponse rise to 90% in 25 s, while it took longer time (50 s) to recover the initial value with turning off the UV light (light illumination (20 mW cm −2 ) at room-temperature). In terms of gas sensing, the influence of the laser treatment on the response of the devices was investigated considering exposure to 100 ppm of ethanol at room temperature (25 °C). The highest response to 100 ppm ethanol vapor was measured for the sample treated at 0.17 J cm −2 , then the response decreases as the laser fluence increases, which was attributed to structural changes of the active layer. Bending experiments were carried out, and for both sensors, the response showed negligible effect over the mechanical strain (Fig. 6b). Table 1 summarizes the reported flexible gas sensors using different TiO 2 nanostructures/thin films, as well as coupling with other materials and forming heterojunctions, together with main features and gas sensing results. As this review is focused on flexible devices, it is indicated the type of substrate used in each study.

Flexible TiO 2 UV sensors/photodetectors
The interest in TiO 2 as sensing/active layer on UV sensors/photodetectors has increased lately, especially when dedicated to flexible and wearable sensing devices. In fact, the integration of nanostructured TiO 2 materials in such devices takes the advantage of its enhanced chemical and mechanical stability, and its wide band gap make this material highly photoactive and stable under UV radiation [27]. Several studies developed flexible TiO 2 UV sensors using different substrates, including paper, plastic, and metallic foils.

TiO 2 nanostructures/thin films for UV sensing
Nunes et al. [14] reported production of flexible nanostructured TiO 2 UV photodetectors using bacterial nanocellulose (BNC), tracing paper and a polyester film as flexible substrates. The selected substrates had in common that they were inexpensive, flexible, recyclable, lightweight, and when associated to low temperature synthesis and absence of seed layer, they become suitable for several low-cost applications. The low-cost synthesis route used was hydrothermal synthesis assisted by microwave irradiation, at low synthesis temperature (80 ºC). Microwave irradiation resulted in uniformly covered substrates, forming continuous TiO 2 films. Individual nanostructured particles with an undefined structure (ranging from small sized squares to rod-like structures) covered mostly of the cellulose-based substrates and fine nanorod aggregates forming TiO 2 flower-like structures were also observed (higher extent on the BNC material when compared to the paper substrate. Figure 7a-d). On the polyester substrate, these flower-like structures were grown side-by-side appearing as a continuous material formed by densely and closely packed nanorod aggregates (Fig. 7e, f ). The UV devices showed responsivities of 0.33 µA W −1 , 0.16 µA W −1 and 0.07 µA W −1 under the bias voltage of 10 V and at room temperature for the TiO 2 films grown on BNC, tracing paper and polyester substrates, respectively (Fig. 7g). The TiO 2 film grown on bacterial nanocellulose displayed enhanced photosensitivity when compared to the other substrates, which can be associated to a higher contribution from the substrate structure, as nanocellulose displays high surface-to-volume ratio, enhancing the sensor performance. This study demonstrated that the structural characteristics of the TiO 2 films and substrates can directly influence the sensors' UV photodetection. Wang et al. [100] reported a flexible TiO 2 UV photodetector fabricated by electrospinning. In this novel device, well aligned TiO 2 nanowires were collected on a flexible mica substrate and interdigitated platinum electrodes were deposited on its surface. The photocurrent of the device could reach up to 47 nA and 38 nA under 254 nm and 365 nm UV light at an applied voltage of 10 V, corresponding to a photo-dark current ratio of 760 and 600, respectively. Moreover, it has been demonstrated that these devices display high mechanical flexibility and durability.

TiO 2 -coupled nanostructures/thin films for UV sensing
As in gas sensors, and in order to overpass the TiO 2 drawbacks and enhance the sensor performance, it has been reported coupling with noble metallic or non-metallic materials, semiconductors and conducting polymers [27,56]. Zhou et al. [101] demonstrated a novel photodetector based on a TiO 2 /graphene hybrid material. The device was fabricated by directly spraying TiO 2 /graphene oxide (TiO 2 /GO) solution on a gold interdigital electrode with subsequent chemical reduction of GO. A flexible Parylene-C was used as substrate. The sensor responsivity achieved 0.482 A W −1 at 3 V bias with 330 nm UV irradiation, in which the rise/recovery times were estimated to be 0.7 s and 0.5 s, respectively. The sensor was compared to pure TiO 2 , and it has been stated that the TiO 2 /graphene hybrid exhibits much higher performance due to the large surface area and prevention of electron-hole recombination brought by graphene. Bending experiments were carried out, and the photocurrent value under bending is 1.13 μA, which shows a 25% decline compared with the unbent condition.
Zheng et al. [102] reported the production of flexible fibrous photodetectors based on a heterojunction of TiO 2 / P3HT (poly(3-hexylthiophene)) prepared by a combination of anodization process and vacuum dip-coating methods. Au nanoparticles were also added to increase the UV sensing performance. The responsivity of the TiO 2 /P3HT-based sensor at 350 nm and 0 V was estimated to be 0.037 mA W −1 , detectivity of 1.63 × 10 10 Jones, and rise and fall times of 0.72 and 0.5 s, respectively. When adding Au, the responsivity at 350 nm and 0 V increased to 0.25 mA W −1 , with a detectivity of 2.9 × 10 10 Jones, and rise and fall times of 0.48 and 2.12 s, respectively.
In Ref. [103], it has been reported UV photodetectors based on free-standing flexible TiO 2 nanofibrous membranes (NFMs) produced by electrospinning. The TiO 2 nanofibers were doped with 2 mol% Y 3+ . The 2 mol% Y 3+ -doped NFMbased photodetector exhibited at 3 V and under 350 nm illumination, a responsivity of 4.5 A W −1 , detectivity of 1.6 × 10 11 Jones, and photocurrent of ≈1.6 µA. The sensor also maintained ≈60% of its original photocurrent after bending at ≈145° for more than 20,000 times. Liu et al. [56] reported the production of flexible Fe 2 O 3 /TiO 2 nanofibrous membranes via electrospinning and followed by high-temperature calcination to be applied on photocatalysis and in UV sensing (Fig. 8a-f ). The flexible device reached currents up to 0.97 and 1.97 nA, when irradiated to 365 and 254 nm, respectively, with an applied voltage of 10 V. The dark current was 0.31 nA, which demonstrated a great increase under UV irradiation. UV photodetector revealed an excellent repeatability and fast response ability. Upon the light, the photocurrent increased very sharply and then drastically decreased when the light was off (Fig. 8g).
Flexible UV sensors based on a TiO 2 nanolayer/CuMnO 2 thin film was demonstrated in [104]. The TiO 2 nanolayer was obtained by thermal oxidation of a Ti foil. Depending on the production parameters, different UV sensing behaviours were observed. The best UV sensor reached 2.52 mA in the dark state, and in UV conditions achieved 12.58 mA. It has also been reported a sensibility value of 4.99 (I UV /I DARK )), with a responsivity of 100.6 A W −1 cm 2 , and a high-speed response time of about 1.56 s. In visible light conditions, the best UV sensor reached a responsivity of 1.1 A W −1 cm 2 .
Qiu et al. [105] reported a photoelectrochemical cell based self-powered and wearable UV photodetector system with high stability, high speed, and an innovative visualization feature (Fig. 9a-c). TiO 2 nanotubes produced by anodization process on a flexible Ti foil and Prussian blue on flexible PET/indium-tin oxide (ITO) were selected as the photoanode and electrochromic counter electrode, respectively. The integrated flexible UV photodetectors demonstrated to be stable under several bending cycles and exhibited a visual indication of UV exposure by recyclable color change without external power. With the increase in UV light intensity, the UV sensor displayed gradually changes from dark blue (original color) to transparent. The UV photodetectors showed that as the light intensity increased from 0.2 to 8 mW cm −2 , the absolute photocurrent increased from 0.012 to 0.222 mA. The UV responsivity measured was 78.3 mA W −1 , with ultralow UV light detection limit (< 10 µA cm −2 ), and fast response with a rise time and a decay time of 0.04 and 0.06 s, respectively ( Fig. 9d-g).
Mojtabavi and Nasirian [106] reported flexible self-powered photodetectors based on polyaniline/TiO 2 (PTP) and TiO 2 / Polyaniline (TPP) heterojunctions. Both heterojunctions were deposited on a flexible PET substrate and investigated as UV and visible (VIS) photodetectors. The investigation of the response speed and the two-color lights detection capability under different power densities at zero bias voltage were performed and the results showed the PTP is a suitable candidate for UV-photodetector with a current value and rise time of 24.9 mA and 3.2 s, respectively, while the TPP is an UV sensors based on flexible wire substrates have also been reported. A fiber-shaped device based on p-CuZnS/n-TiO 2 nanotubes produced by the anodization process of a Ti micro-wire is reported in [107] (Fig. 10a). The flexible fiber-shaped UV sensor showed an outstanding responsivity of 640 A W −1 , external quantum efficiency (EQE) of 2.3 × 10 5 %, and photocurrent of ≈ 4 mA at 3 V (Fig. 10b). In addition, it exhibited self-powered characteristics, with a maximum responsivity of Fig. 9 a, b Application of the flexible and wearable UV photodetector on a wrist, with and without UV exposure, evidencing the color change, together with (c) the real images of the flexible device in the dark and exposed to different UV illumination intensities. d Typical I-V curve for the flexible device under dark and UV illumination with various intensities (from 0.2 to 8 mW cm −2   ≈ 2.5 mA W −1 and a fast response speed of < 0.2 s at zero bias. The outstanding optoelectronic performance of this flexible UV sensor was mainly attributed to its unique nanotube array structure that favors for light harvesting and efficient photocarrier collection. Moreover, a real-time wearable UV radiation sensor that reads out ambient UV power density and transmits data to smart phones via Wi-Fi was demonstrated (Fig. 10c). In Ref. [108], Ti microwires were also used for producing UV sensors. In this innovative approach, the chemical oxidation modification method was used on Ti wires (Ø ≈ 800 µm, 100 µm) growing radially TiO 2 nanoneedles/nanoparticles with the anatase crystalline phase. The TiO 2 nanostructures/Ti wire-based UV sensors had their photo-responsive performance investigated as a function of fixed bias voltages (± 1 V and ± 7 V) and under light sources with wavelengths of 365 nm, 405 nm and 535 nm. The non-flexible UV sensor produced with Ø ≈ 800 µm demonstrated higher photo-responsivity ≈35.024 µA W −1 under the light intensity ≈18 mW/cm 2 (λ ≈ 365 nm) at bias voltage ≈ − 1 V, while the flexible UV sensor with Ø ≈ 100 µm reached 1.682 µA W −1 in the same conditions. Other devices were also described with the intention of producing wire-based self-powered sensors, and flexible energy harvesters. Pan et al. [109] reported the production of UV sensors based on TiO 2 -coated ZnO nanotube arrays uniformly grown on flexible molybdenum microwires. Under UV illumination, the responsivity of the UV detectors was about 138.3 μA W −1 cm −1 , while the response time is 31 ms for rising and 79 ms for decaying. Table 2 shows a comparison between the TiO 2 nanostructures/thin films integrated on flexible UV sensors together with some approaches described in literature for enhancing the TiO 2 sensing behaviour. The flexible substrates used are indicated, together with the most relevant parameters for each sensing device.
Different structures and approaches have been described in this review, where most of the reports presented flexible sensors produced and measured in laboratory conditions, far away from the large-scale industrial production. Moreover, the flexible sensing technology still presents some bottlenecks that must be overpassed for its wide utilization in practical applications.

Conclusions and future perspectives
This review summarized the latest developments in flexible gas and UV sensors/photodetectors having nanostructured TiO 2 as the sensing layer. As it has been largely discussed, TiO 2 has some drawbacks to its practical and wide use, nevertheless, several alternatives for surpassing these limitations are under investigation, with promising results as presented in this work. The emphasis of the review was TiO 2 at the nanoscale, as it presents high surface-to-volume ratios and high surface reaction activity, enhancing the sensor performance when compared to bulk counterparts. The working temperatures of the gas and UV sensors are evolving to room temperature, and this review demonstrated, that many approaches for the flexible TiO 2 sensing devices operating at room temperature involved coupling with other elements or materials such as metals, other semiconductors, conducting polymers, carbon-based nanomaterials, but also cellulose-based nanostructures. Many studies revealed that TiO 2 nanotube arrays produced by anodization processes or by thermal oxidation can achieve high responses to the target gases with good selectivity, and this was associated to the high surface-to-volume ratio of such nanostructures.
Miniaturization of sensors maintaining or increasing their sensitivity and selectivity is one of the main targets of sensing technology, and for flexible devices, it is critical to improve resolution of printing techniques, reaching the nanoscale with accuracy but also allowing mass-production for the widely use of such nanodevices. Another aspect is the fully integration of green production technologies, i.e. wet-chemical synthesis, which will drop expressively the final sensor cost, while protecting the environment. But in that sense, it is still required deep investigation to guarantee reproducibility and chemical and mechanical stabilities, but also avoid contamination from the synthesis reagents and structural defects that can be imposed by the production route.
In this review, the advantages of using flexible substrates have been highlighted, and the main goal is to produce inexpensive, lightweight, highly adaptable, disposable, and recyclable devices, compatible with wearable technologies, to produce the next generation IoT smart sensing devices. However, to reach the needs of practical applications, it is imperative to combine multiple sensors to originate an integrated IoT system. Hence, in terms of future work, systematic studies on improving the sensing layers, printing methods, and development of scalable and routine processes are necessary for reaching the use of sensors in large-scale guaranteeing the well-being and comfort to the final users, while reducing electronic waste.