Multi-Sensing Platform Based on 2D Monoelement Germanane

RESEARCH ARTICLE www.advmat.de Multi-Sensing Platform Based on 2D Monoelement Germanane Jayraj V. Vaghasiya, Carmen C. Mayorga-Martinez, Keval K. Sonigara, Petr Lazar, and Martin Pumera* Covalently functionalized germanane is a novel type of fluorescent probe that can be employed in material science and analytical sensing. Here, a fluorometric sensing platform based on methyl-functionalized germanane (CH3Ge) is developed for gas (humidity and ammonia) sensing, pH (1–9) sensing, and anti-counterfeiting. Luminescence (red–orange) is seen when a gas molecule intercalates into the interlayer space of CH3Ge and the luminescence disappears upon deintercalation. This allows for direct detection of gas absorption via fluorometric measurements of the CH3Ge. Structural and optical properties of CH3Ge with intercalated gas molecules are investigated by density functional theory (DFT). To demonstrate real-time and on-the-spot testing, absorbed gas molecules are first precisely quantified by CH3Ge using a smartphone camera with an installed color intensity processing application (APP). Further, CH3Ge-paper-based sensor is integrated into real food packets (e.g., fish and milk) to monitor the shelf life of perishable foods. Finally, CH3Ge-based rewritable paper is applied in water jet printing to illustrate the potential for secret communication with quick coloration and good reversibility by water evaporation. 1. Introduction In recent years, 2D-based graphene analogs have opened up fascinating opportunities for basic and applied research in diverse field applications such as biomedical, environmental, and electronic devices.[1–7] Germanane has attracted a lot of interest as a unique electronic material because it can tune the J. V. Vaghasiya, K. K. Sonigara, M. Pumera Future Energy and Innovation Laboratory Central European Institute of Technology Brno University of Technology Purkyˇ nova 123, Brno 61200, Czech Republic E-mail: [email protected].cz J.V.Vaghasiya,C.C.Mayorga-Martinez,M.Pumera CenterforAdvancedFunctionalNanorobots DepartmentofInorganicChemistry Faculty ofChemicalTechnology UniversityofChemistryandTechnologyPrague Technická5,Prague16628,CzechRepublic The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adma.202304694 © 2023 The Authors. Advanced Materials published by Wiley-VCH GmbH. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited. DOI: 10.1002/adma.202304694 morphological and physical properties by changing the nature of the covalently terminated ligand.[8–12] Typically, this material can be produced by exfoliation of CaGe2(also known as the Zintl phase) in an acidic media. Afterward, topochemical deintercalation of CaGe2with alkyl halide yields alkyl germanane, that is, hydrogen or methyl germanane.[13,14] Germanane has recently been presented as a novel active material with tailored properties for energy storage,[15,16] sensing (chemical and biological),[17,18] optoelectronic,[19,20] hydrogen storage,[21,22] energy conversion, and photo-electrocatalysis.[23,24] The fine-tuning of germanane properties (i.e., bandgap and physical properties) can be achieved by the covalent surface termination of germanane. Recently, our research group tuned the photocatalytic properties of germanane by altering various organic functional groups.[14] When the methyl functional group replaced hydrogen on the surface of germanane, it enhanced its thermal stability and widened the bandgap.[25] Moreover, CH3Ge has been employed as a biosensor to identify liver cirrhosis biomarkers and glucose.[17,18] However, germanane-based biosensors are still in their infancy and require more research. CH3Ge also demonstrates outstanding band-edge fluorescence and photoluminescence (PL) properties.[26–28] Therefore, P. Lazar Regional Centre of Advanced Technologies and Materials Czech Advanced Technology and Research Institute (CATRIN) Palacký University Olomouc Šlechtitel˚ u 27, Olomouc 779 00, Czechia M. Pumera Faculty of Electrical Engineering and Computer Science VSB – Technical University of Ostrava 17. listopadu 2172/15, Ostrava 70800, Czech Republic M. Pumera Department of Medical Research China Medical University Hospital China Medical University No. 91 Hsueh-Shih Road, Taichung 40402, Taiwan M. Pumera Department of Chemical and Biomolecular Engineering Yonsei University 50 Yonsei-ro, Seodaemun-gu, Seoul 03722, South Korea Adv. Mater. 2023,35, 2304694 2304694 (1 of 13) © 2023 The Authors. Advanced Materials published by Wiley-VCH GmbH www.advancedsciencenews.com www.advmat.de CH3Ge has been documented as a fluorescent marker on a microrobot system for drug delivery.[29] On the other hand, the PL of CH3Ge is highly associated with the absorption of water molecules in its interlayer space. The study reveals that when CH3Ge is hydrated, it exhibits reddish–orange luminescence but when it is dry the luminescence is invisible. But owing to a lack of germanane-based sensor engineering knowledge in the research community, it is yet to be investigated as a real-scale application. The multi-stacked compact structure of CH3Ge makes it challenging to integrate in sensing devices with precise interfaces. Hence, here we demonstrated material engineering to develop a multifunctional paper-based sensor from CH3Ge. CH3Ge’s buckled honeycomb structure with sp3hybridization could help increase its absorption of gas molecules (humidity) and chemical reactivity. Due to its unique structural and PL properties, it is conceivable to exploit CH3Ge to detect other volatile gases in the atmosphere such as water vapor, and acidic and basic gases. This would be a new direction for the development of CH3Ge-based gas sensors. Graphene-analogous 2D materials, such as transition metal dichalcogenides (e.g., MoS2,MoSe 2,andWS 2),[30–32] transition metal carbides (e.g., MXene Ti3C2Tx),[33,34] hexagonal boron nitride,[35] and black phosphorous[36] have become popular recent years for gas sensing. Functionalized germanane also has outstanding capabilities to sense a broad range of harmful gases such as NH3,SO 2,andNO 2.[37] Unfortunately, reported state-ofart devices for gas sensing applications rely on electrochemical workstations that are fully dependent on power sources and not suitable for on-site and real-time applications.[29–35,37] Therefore, it is critically important to develop quick, instrument-free, and rapidly sensitive sensors for the effective detection of a variety of gases. Additionally, materials that change color in response to chemical inputs (gas, pH, and solvent) act as the primary components in biometric security and secret communication applications. Recently developed rewritable imaging technologies have been functionalized for water jet rewritable papers, self-erasing images, and photochromic paper.[38,39] The majority of materials used in this technology are based on organic materials (polymers, dyes, and pigments) and inorganic materials (perovskite).[40–42] Nevertheless, the technology is not eco-friendly enough due to the complex synthesis process of materials and the erasing solvents are either flammable or only available from specific chemical vendors.[43,44] Thus, the development of rewritable paper using new materials in an environmentally friendly manner is essential. Here, we present a first-time simple and straightforward CH3Ge fluorometric sensing platform that can be used for gas and pH detection and secret communication. The absorbed gases can be identified based on their particular luminescence response (redor blueshift) through the intercalation of gases into the CH3Ge van der Waals gap. The extent of absorbed gases affects PL intensity, making it possible to directly monitor gas absorption by quantifying the luminescence intensity of the CH3Ge. Smartphone-based fluorometric sensors have received a lot of interest recently due to easily quantified color changes in the sample by using a smartphone application.[45,46] Specifically, the amount of gas absorbed by CH3Ge was measured using a smartphone camera enabled with the Color Grab color intensity processing application to measure changes to the R (red)/G (green)/B (blue) color components. This smartphoneassisted fluorometric platform could provide quick and on-site analysis without the use of complicated instruments. To demonstrate the application, the CH3Ge-paper-based sensor was used to monitor the freshness of real food (i.e., fish and milk). Another important application has been illustrated by printing confidential information on CH3Ge-based paper using a water inkjetprinting technique, where water is used as a secret ink source to hide symbolic or textual information on top-secret documents. Most importantly, once such a document is read, the information can be easily removed by simple water evaporation. This is the first time demonstrating the use of CH3Ge for real-world applications such as humidity monitoring, food quality analysis, and secret communication. To the best of our knowledge, the mono-element 2D materials-based water inkjet rewritable paper has never been reported for secret communication applications. 2. Result and Discussion In the family of layered van der Waals 2D materials, CH3Ge is one of the fascinating materials with strong PL properties associated with water interaction. Here, we develop for the first timeaCH 3Ge-based fluorometric sensing platform that can be used for the detection of gas, pH, and secret communication. The PL intensity of CH3Ge is highly dependent on the pH of solvent that is intercalated into the interlayer spacing of CH3Ge and emits light accordingly. The intensity of PL is triggered by the intercalated molecules owing to local structural distortion of the CH3Ge caused by the dative interaction of water molecules with the Ge─C. Taking benefit of this property, we focused on the detection of different gases (humidity and ammonia) and pH of water (1–9). The absorbed gases can be identified based on the particular color response, that is, acidic gases emit red light and basic gases emit red–orange light.[27] Besides, luminescence intensity is influenced by the absorbed gases. This color change ability can be used to evaluate both gases (relative humidity and ammonia) and the freshness of food (Figure 1a,b). For instance, ammonia gas produced by spoiling fish and lactic acid generated by spoiling milk[23] could be easily measured using a CH3Ge sensor. After that, the amount of gases absorbed by CH3Ge was evaluated using a smartphone camera and color intensity processing software to quantify the color change with RGB color components (Figure 1d). Taking the benefit of hydrochromic properties, CH3Ge has been used for secret communication as well. Here, we show a rapid and facile approach for the preparation of a communication medium by applying CH3Ge materials to the surface of ordinary printing paper; after that, CH3Ge-coated paper could be printed multiple times using water as an ink (Figure 1c). Interestingly, when the CH3Ge paper comes in contact with water, the printed information appears upon exposure of UV light and then disappears when the water evaporates. Concerns about an increase in leaks of confidential information and counterfeiting indicate an opportunity for anti-counterfeiting and encrypted communications as illustrated herein. Before evaluating CH3Ge sensor performance in gases and pH detection, it is critical to first investigate the morphology and structural properties of CH3Ge. Adv. Mater. 2023,35, 2304694 2304694 (2 of 13) © 2023 The Authors. Advanced Materials published by Wiley-VCH GmbH 15214095, 2023, 44, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adma.202304694 by Technical University In Brno, Wiley Online Library on [21/02/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.advmat.de Figure 1. Smartphone-assisted fluorometric platform using CH3Ge-based paper for sensing and secret communication. a) CH3Ge-based gas sensors for humidity monitoring and food quality analysis (by ammonia sensing). b) CH3Ge-based pH-responsive sensor for food quality analysis (by lactic acid sensing). c) Schematic illustration of CH3Ge-based rewritable paper with water jet printing for anti-counterfeiting and encrypted communications. d) Color intensity was determined using a smartphone module. 2.1. Morphological, Structural, and Optical Properties Figure 2a shows a schematic representation of the synthesis of CH3Ge and the fabrication of the CH3Ge-based sensor. In a nutshell, CH3Ge was prepared by the topotactic deintercalation of CaGe2with methyl iodide in an organic solvent.[14,18] The obtained multilayer CH3Ge was exfoliated by the liquid phase exfoliation method[11] and CH3Ge flakes were then spray-coated onto cellulose paper for further use. A more detailed description of the synthesis and spray coating procedure is given in the Experimental Section. The morphology of prepared samples was investigated by scanning electron microscopy (SEM) and energydispersive X-ray spectroscopy (EDS). The multilayer structure of CH3Ge is clearly visible in the SEM image (Figure 2b). The magnified SEM image (Figure 2c) reveals an enlarged lamellar structure, indicating that calcium was successfully removed. After ultrasonic treatment, CH3Ge has a nanoflake-like morphology with lateral sizes in the micrometer range (Figure 2d). The scanning transmission electron microscopy (STEM) image also depicts a transparent flake-like structure for CH3Ge, noting good delamination. Atomic force microscopy topography image shows that the delaminated CH3Ge flakes thickness of 4.2 ±0.5 nm (Figure S2, Supporting Information), which is comparable with previously published reports.[47] EDS analysis (Figure 2e,f) Adv. Mater. 2023,35, 2304694 2304694 (3 of 13) © 2023 The Authors. Advanced Materials published by Wiley-VCH GmbH 15214095, 2023, 44, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adma.202304694 by Technical University In Brno, Wiley Online Library on [21/02/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.advmat.de Figure 2. Preparation and morphology study of CH3Ge. a) Synthesis procedure of CH3Ge and spray-coated CH3Ge-based paper. b,c) Lowand highresolution SEM images of bulk CH3Ge. d) STEM image of CH3Ge flakes. e,f) EDS images of CH3Ge flakes. g) SEM images of CH3Ge-coated paper. reveals that Ge and C are distributed uniformly throughout the flakes, confirming the success of methyl functionalization on germanane. Figure 2g shows SEM images of CH3Ge-decorated cellulose paper. The paper fabric has a randomly oriented fibrous structure and a clean surface as shown in Figure S1a, Supporting Information. In contrast, CH3Ge-decorated paper (Figure 2g) shows a rough surface and CH3Ge flakes can be seen on the surface of paper fiber and gaps. High-resolution SEM image (inset of Figure 2g) and EDS mapping (Figure S1b–e, Supporting Information) indicate that fibers were uniformly covered by the CH3Ge flakes. Further CH3Ge optical and structural studies in dried and hydrated conditions were conducted to investigate the PL color change mechanism. In humid conditions, CH3Ge-decorated paper quickly changes PL color from white to red upon 365 nm light irradiation (Figure 3a). Figure 3b depicts PL spectra in both dried and hydrated states, demonstrating that PL intensity increases when water molecules are absorbed. Previous research found that the intercalation of water molecules into the interlayer space of CH3Ge causes above-bandgap luminescence.[27] The intensity of the PL will also change depending on the relative humidity (RH) levels. Further, the luminescence intensity was converted to the chromaticity values xand y.Figure3cdepicts the color change after exposure to humidity. The Commission internationale de l’éclairage (CIE) coordinates change from x=0.586, y=0.412 in dried CH3Ge to x=0.626, y=0.372 in hydrated CH3Ge. The X-ray diffraction (XRD) pattern (Figure 3d) in dried-state CH3Ge exhibits a highly crystalline phase that Adv. Mater. 2023,35, 2304694 2304694 (4 of 13) © 2023 The Authors. Advanced Materials published by Wiley-VCH GmbH 15214095, 2023, 44, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adma.202304694 by Technical University In Brno, Wiley Online Library on [21/02/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.advmat.de Figure 3. Structural and photophysical properties of CH3Ge. a) Photograph of CH3Ge-decorated paper to change color in humid conditions upon 𝜆ex =365 nm. b) PL spectra (𝜆ex =365 nm) of CH3Ge in dried and hydrated states. c) CH3Ge colors in CIE 1931 color space. d) XRD pattern of dried and hydrated CH3Ge. e) ATR-FTIR spectra of CH3Ge recorded at a different time in 30% humidity. f) DFT calculations of CH3Ge interacting with water molecules (germanane: light gray; carbon; dark gray; hydrogen: white; oxygen: red). is consistent with previously published reports.[25] In contrast, the high-intensity diffraction peak (002) changes from 9.661 to 9.850 Å upon exposed humidity due to the fact that water molecules could enter the interlayer space and broaden the layers. Further, FTIR spectroscopy was used to investigate CH3Ge water absorption (30% humidity) properties as a function of time. As shown in Figure 3e, the CH3Ge absorption band at 3299 cm−1 increases as a function of time. This is due to the stretching vibration peak created by ─OH when water is absorbed by CH3Ge. Ultimately, long-time humidity exposure raises the number of ─OH groups by increasing bound water molecules on CH3Ge. Therefore, all humidity experiments were carried out in constant time intervals. The typical absorption peaks at 2898 and 580 cm−1 are responsible for ─CH3and Ge─C stretching vibrations, respectively. Peaks at 1403 and 1226 cm−1are attributed to ─CH3 bending vibration while peaks at 758 cm−1are ascribed to ─CH3 rocking vibration.[14,25] A slight difference observed at ≈700 cm−1, indicating enhanced hydrogen bonding between water molecules and CH3Ge. We performed density functional theory (DFT) calculations of CH3Ge interacting with water molecules to gain further insight into the characteristics of the structural and optical response. We Adv. Mater. 2023,35, 2304694 2304694 (5 of 13) © 2023 The Authors. Advanced Materials published by Wiley-VCH GmbH 15214095, 2023, 44, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adma.202304694 by Technical University In Brno, Wiley Online Library on [21/02/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.advmat.de consider fully functionalized bulk CH3Ge in the 1T primitive cell as the computational model (Figure 3f). The calculated lattice parameters of the fully relaxed primitive cell are a=3.98 and c=8.01 Ǻ. We note that the calculated value of the clattice parameter is smaller than that obtained from powder XRD due to the interlayer turbostratic disorder and curvature, which are often present in the synthesized CH3Ge flakes. Next, we add a water molecule intercalated in between the layers. Notice that we intercalate water in the layers of the bulk primitive cell so the model corresponds to the fully intercalated water layer because the water molecule is periodically repeated in the xy (in-plane) direction. The oxygen atom of a water molecule creates the hydrogen bond to the hydrogen of the CH3group (the O─H bond length of 2.57 Ǻ), making the intercalation process thermodynamically exothermic, that is, the formation energy for intercalation is negative (−0.03 eV). Intercalation of water increases the clattice constant to 10.3 Ǻ. This value should be considered as the upper boundary of the lattice expansion because it is calculated for the fully intercalated water layer. The structure of CH3Ge does not undergo any notable deformation upon water intercalation (apart from the increase of the interlayer spacing); the bond lengths of the C─Ge and C─H bonds are unchanged and the methyl group next to the water molecule is not deformed or tilted. We calculated the true optical bandgap using the time-dependent DFT (TD-DFT) method with hybrid-functional HSE06 as the kernel. The optical gap is the lowest energy dipole-allowed transition observed in the absorption spectra and is due to excitonic effects generally lower than the electronic gap, which is commonly calculated as the energy difference between the DFT energy levels. The calculated optical band gap of pure bulk CH3Ge is 1.76 eV while water intercalation increases the bandgap to 2.01 eV. No mid-gap state emerges upon water intercalation and the bandgap remains as the direct gap, located at the Γpoint of the first Brillouin zone. Thus, the intercalation blueshifts the optical response into the red part of visible light in agreement with experimental spectra. Asel et al. studied the mechanism of water intercalation into CH3Ge and demonstrated that the intense above-gap photoluminescence was caused by a local structural distortion of the Ge framework due to the dative bonding to intercalated water molecules.[27] 2.2. CH3Ge-Paper-Based Gas-Responsive Sensor Based on the unique interaction of CH3Ge with water molecules and color-changing properties, CH3Ge-decorated paper can be utilized as a humidity sensor. Although the emission color in presence of humidity could be recognized visually, it could not achieve favorable accuracy by human eyes due to incapability to distinguish monochromatic intensity change. In general, such detection still depends on sophisticated and bulky laboratory instruments that are time-consuming, necessitate qualified manpower to operate, and are not appropriate for on-site or real-time detection. Therefore, the intensity of the CH3Ge-decorated paper as a function of RH was measured using a smartphone and analyzed using the Color Grab application (Figure 4a). The smartphone and CH3Ge paper were placed ≈15 cm apart and perpendicular to one another, with the incident 365 nm light providing homogeneous illumination at 60°. To achieve a better humidity response of the CH3Ge-decorated paper, the amount of active materials loaded on the substrate must be optimized. Here, we spray-coated three different amounts of CH3Ge flakes onto a paper substrate: 0.3 mg cm−2(sample 1), 0.65 mg cm−2(sample 2), and 1 mg cm−2(sample 3) (Figure 4b). The color intensity change of the CH3Ge-decorated paper during the 30% humidity exposure can be seen visually in the inset of Figure 4b. We can also say the paper becomes redder as the loading of the active material increases. A smartphone can be utilized to view the exact value of color intensity change. Especially in comparison with spectrofluorometers, the use of a mobile device as a color intensity detector has been defined as one of the fastest and simplest tools. In grayscale percentages, the mean intensity of different loading samples 1, 2, and 3 are 37.66% (±1.45), 46% (±2.3), and 55.3% (±1.45), respectively. When an excess amount of CH3Ge (>1mgcm −2) is coated on paper causes cracking, increased surface non-informativity, and nanosheet aggregation, all of which reduce sensing performance. Thus, we decided to undertake all subsequent humidity and pH sensing studies with optimum sample 3. In order to demonstrate the CH3Ge-decorated paper’s ability to change color intensity upon various RH levels, the R/G/B values of CH3Ge papers were measured between RH 0% and RH 100% using a smartphone module. Figure 4d (green color square) depicts original photographs of CH3Ge paper at various humidity levels. It can be challenging to distinguish color changes with our naked eyes. The obtained different color intensity shades processed by the Color Grab smartphone application were replicated to create a solid color as depicted in Figure 4e (black-colored square). Figure 4c shows that the R-value rises from 148 in RH 10% to 216 in RH 100% while a subtle difference is found in G and B values. A similar trend was also observed in the grayscale percentage (Figure 4f). This investigation successfully revealed the hues and their related intensities visually in the form of grayscale percentage and RGB value. The acquired trend as a function of RH was further validated by a spectrophotometer (Figure 4g). It clearly depicts the increase in PL intensity at various RH levels. Similar trends for fluorescence detection were also seen on the smartphone sensing platform without the use of additional instruments. Further, we demonstrated a prototype mono color change CH3Ge-based humidity indicator card that can detect the entire range (Figure 4h). This card can be utilized in food/medicine packaging applications to provide a visual cue when the relative humidity is increasing. There are many different kinds of commercial humidity indicator cards available and some provide benefits such as low cost, ease of use, and long lifespan. However, their low sensitivity and detection range remain major issues. Figure 4h clearly shows the color change variation with different RH. Unfortunately, this card only provided a range of humidity levels. For these reasons, we used a smartphone platform to measure accurately and precisely the humidity in real time. To examine the solvent selectivity of the developed CH3Gepaper-based sensor, RGB values were recorded after the addition of methanol, acetone, ethanol, chloroform, dichloromethane, isopropyl alcohol, and water. Figure 5a depicts RGB values of the CH3Ge sensor in different solvent vapor. The RGB value of CH3Ge sensors in water vapor is the greatest, revealing high specificity in comparison to all other solvents. However, the Adv. Mater. 2023,35, 2304694 2304694 (6 of 13) © 2023 The Authors. Advanced Materials published by Wiley-VCH GmbH 15214095, 2023, 44, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adma.202304694 by Technical University In Brno, Wiley Online Library on [21/02/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.advmat.de Figure 4. CH3Ge-based humidity-responsive sensor. a) Schematic representation of color intensity quantification using a smartphone platform equipped with the Color Grab APP. b) Mean intensity of different amounts of CH3Ge loading on a paper substrate. c) Mean RGB value as a function of humidity. d) Real images in different humidity levels captured by a smartphone camera. e) Color Grab APP-generated duplicated images. f) Average grayscale percentage in different humidity levels. g) PL spectra as a function of humidity. h) Mono-color-change of CH3Ge-based humidity indicator. slightly higher value observed in ethanol compared to other organicsolventsisduetoits>1.0% water content. The detection of water or trace levels of humidity from commercial organic solvents will be made easier by this sensor. Furthermore, the limit of detection (LOD) of the CH3Ge sensor was calculated using the increasing luminescence intensity at different RH. As shown in Figure 5b, the LOD of the CH3Ge-paper-based humidity sensor was as low as 9.32% RH. Furthermore, we use CH3Ge paper to detect ammonia gas. Figure 5c inset depicts a noticeable variation following ammonia gas exposure on CH3Ge, which results in an emission that appears more orange. The red color value found around 189 using the color intensity processing software Color Grab. To understand this color shift, we calculated the optical bandgap for CH3Ge intercalated by ammonia. The TD-DFT value of the optical bandgap is 2.11 eV, higher than the value for intercalated Adv. Mater. 2023,35, 2304694 2304694 (7 of 13) © 2023 The Authors. Advanced Materials published by Wiley-VCH GmbH 15214095, 2023, 44, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adma.202304694 by Technical University In Brno, Wiley Online Library on [21/02/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.advmat.de Figure 5. CH3Ge-based ammonia responsive sensor. a) Solvent vapor selectivity with CH3Ge-decorated paper. b) Low limit of detection (LOD) plot of CH3Ge sensor. c) R-value as a function of ammonia exposure. d) RGB values of CH3Ge in fresh and spoiled fish. Inset: digital images of CH3Ge papers in fresh fish (green square) and spoiled fish (black square). e) Schematic representation of color change of CH3Ge paper in relation to fish spoilage and quantification using a smartphone with color intensity processing software, Color Grab. water calculated with the same computational parameters. Thus, this increase explains the shift to more orange color. Intercalation of ammonia is also exothermic: the formation energy is −0.038 eV, similar to that of water. We also investigated exposure to ammonia gas at various time intervals on CH3Ge and the results are shown in Figure 5d. Following that, fish meat freshness and spoilage were examined in relation to the package environment shifting to an alkaline condition. After 5 days of storage at room temperature, the color of CH3Ge paper changes from red to light orange (Figure 5e). Using Color Grab software to process color intensity, the red color value increased from 161 to 186, indicating a more alkaline package environment (Figure 5i). The volatile basic gases generated from spoiled fish may have been a major factor in changing the color of CH3Ge paper. Ammonia compounds (e.g., diand trimethylamine) are a common by-product of spoiled fish due to protein decomposition by enzymes and bacteria.[48] These results indicate that CH3Ge has a good response to ammonia gases and can imply the quality of foods in the beginning phases of spoilage. 2.3. CH3Ge-Paper-Based pH-Responsive Sensor After successfully demonstrating the changing color intensity of CH3Ge in different gas environments, we found that the emission energy of CH3Ge varies depending on the pH. pH is an important parameter in many fields, including food testing, medical diagnosis, and soil monitoring.[49,50] The development of unique visual and quantitative pH measuring platforms remains critical for some particular on-site application fields (e.g., food analysis). Therefore, we fabricated a CH3Ge-paper-based pH sensor. The protonated lone pairs on water molecules removed the native interaction between the oxygen and germanane, which could cause color changes at various pH.[27] Hence, the performance of a color-changing CH3Ge-based pH sensor was thoroughly Adv. Mater. 2023,35, 2304694 2304694 (8 of 13) © 2023 The Authors. Advanced Materials published by Wiley-VCH GmbH 15214095, 2023, 44, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adma.202304694 by Technical University In Brno, Wiley Online Library on [21/02/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.advmat.de Figure 6. CH3Ge-based pH-responsive sensor. a) Images of the pH color change CH3Ge paper captured with a smartphone camera. b) RGB values at different pH. c) PL spectra as a function of pH. d) DRS in pH 1, 7, and 11. e) RGB values of CH3Ge in fresh and spoiled milk. f) Schematic representation color change of CH3Ge paper in relation to milk spoilage and quantification using a smartphone with Color Grab color intensity processing software. investigated. As shown in Figure 6a, the color of CH3Ge gradually changes as pH increases. When pH levels are in acidic and basic ranges, the CH3Ge changes from dark red to orange, respectively. These visible color changes aid users in determining the estimated pH of the sample using their naked eyes and without any help from instruments. However, the perception of color differences varies from person to person, and some viewers may have a color vision deficiency. These obstacles could be overcome by using a smartphone equipped with a color intensity processing application as we have demonstrated here. The pH range was quantified by measuring the color parameter (RGB value) from images captured with a smartphone camera using the Color Grab application (Figure 6b). The red color intensity was a function of increasing pH while subtle differences were observed in the blue and green color values. Figure 6c depicts the PL spectra of CH3Ge at different pH levels. The red light emission in pH 1, 3, and 5 is ≈1.977 eV (627 nm) while 1.971 eV (629 nm) was observed in pH 7. At pH 9 and 11, the blueshift of the CH3Ge was found at 1.990 (623 nm) and 1.996 eV (621 nm), respectively. The observed blueshift is probably attributable to partial decomposition, which will generate Ge(OH)2.[27] Multiple pH sensing cycles of CH3Ge paper were performed at pH 9 and pH 11. There is no significant change observed in sensing performance at pH 9, whereas a dramatic change was observed at pH 11 (Figure S3, Supporting Information). This indicates that a CH3Ge-based sensor can function below pH 11 for real-world applications. Furthermore, the variation in band edge as a function of pH was recorded using diffuse reflectance spectroscopy (DRS). The bandgap was calculated from DRS using the Kubelka–Munk function. Figure 6d shows a clear difference in band edge with Adv. Mater. 2023,35, 2304694 2304694 (9 of 13) © 2023 The Authors. Advanced Materials published by Wiley-VCH GmbH 15214095, 2023, 44, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adma.202304694 by Technical University In Brno, Wiley Online Library on [21/02/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License