Multi‐Sensing Platform Based on 2D Monoelement Germanane

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.


Introduction
[3][4][5][6][7] Germanane has attracted a lot of interest as a unique electronic material because it can tune the CH 3 Ge has been documented as a fluorescent marker on a microrobot system for drug delivery. [29]On the other hand, the PL of CH 3 Ge is highly associated with the absorption of water molecules in its interlayer space.The study reveals that when CH 3 Ge 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 CH 3 Ge 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 CH 3 Ge.CH 3 Ge's buckled honeycomb structure with sp 3 hybridization 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 CH 3 Ge 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 CH 3 Ge-based gas sensors.
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]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 CH 3 Ge 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 (red-or blueshift) through the intercalation of gases into the CH 3 Ge 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 CH 3 Ge.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 CH 3 Ge 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 CH 3 Ge-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 CH 3 Ge-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 CH 3 Ge 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.

Result and Discussion
In the family of layered van der Waals 2D materials, CH 3 Ge is one of the fascinating materials with strong PL properties associated with water interaction.Here, we develop for the first time a CH 3 Ge-based fluorometric sensing platform that can be used for the detection of gas, pH, and secret communication.The PL intensity of CH 3 Ge is highly dependent on the pH of solvent that is intercalated into the interlayer spacing of CH 3 Ge and emits light accordingly.The intensity of PL is triggered by the intercalated molecules owing to local structural distortion of the CH 3 Ge 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 CH 3 Ge sensor.After that, the amount of gases absorbed by CH 3 Ge 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, CH 3 Ge 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 CH 3 Ge materials to the surface of ordinary printing paper; after that, CH 3 Ge-coated paper could be printed multiple times using water as an ink (Figure 1c).Interestingly, when the CH 3 Ge 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 CH 3 Ge sensor performance in gases and pH detection, it is critical to first investigate the morphology and structural properties of CH 3 Ge.

Morphological, Structural, and Optical Properties
Figure 2a shows a schematic representation of the synthesis of CH 3 Ge and the fabrication of the CH 3 Ge-based sensor.In a nutshell, CH 3 Ge was prepared by the topotactic deintercalation of CaGe 2 with methyl iodide in an organic solvent. [14,18]The obtained multilayer CH 3 Ge was exfoliated by the liquid phase exfoliation method [11] and CH 3 Ge 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 energy-dispersive X-ray spectroscopy (EDS).The multilayer structure of CH 3 Ge 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, CH 3 Ge 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 CH 3 Ge, noting good delamination.Atomic force microscopy topography image shows that the delaminated CH 3 Ge 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) 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 CH 3 Ge-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, CH 3 Ge-decorated paper (Figure 2g) shows a rough surface and CH 3 Ge 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 CH 3 Ge flakes.
Further CH 3 Ge optical and structural studies in dried and hydrated conditions were conducted to investigate the PL color change mechanism.In humid conditions, CH 3 Ge-decorated pa-per 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 CH 3 Ge causes above-bandgap luminescence. [27]he intensity of the PL will also change depending on the relative humidity (RH) levels.Further, the luminescence intensity was converted to the chromaticity values x and y. Figure 3c depicts 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 CH 3 Ge to x = 0.626, y = 0.372 in hydrated CH 3 Ge.The X-ray diffraction (XRD) pattern (Figure 3d) in dried-state CH 3 Ge exhibits a highly crystalline phase that 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 CH 3 Ge water absorption (30% humidity) properties as a function of time.As shown in Figure 3e, the CH 3 Ge 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 CH 3 Ge.Ultimately, long-time humidity exposure raises the number of ─OH groups by increasing bound water molecules on CH 3 Ge.Therefore, all humidity experiments were carried out in constant time intervals.The typical absorption peaks at 2898 and 580 cm −1 are responsible for ─CH 3 and Ge─C stretching vibrations, respectively.Peaks at 1403 and 1226 cm −1 are attributed to ─CH 3 bending vibration while peaks at 758 cm −1 are ascribed to ─CH 3 rocking vibration. [14,25]A slight difference observed at ≈700 cm −1 , indicating enhanced hydrogen bonding between water molecules and CH 3 Ge.
We performed density functional theory (DFT) calculations of CH 3 Ge interacting with water molecules to gain further insight into the characteristics of the structural and optical response.We consider fully functionalized bulk CH 3 Ge 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 c lattice parameter is smaller than that obtained from powder XRD due to the interlayer turbostratic disorder and curvature, which are often present in the synthesized CH 3 Ge 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 CH 3 group (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 c lattice 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 CH 3 Ge 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 CH 3 Ge 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 Γ point 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 CH 3 Ge 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]

CH 3 Ge-Paper-Based Gas-Responsive Sensor
Based on the unique interaction of CH 3 Ge with water molecules and color-changing properties, CH 3 Ge-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 CH 3 Ge-decorated paper as a function of RH was measured using a smartphone and analyzed using the Color Grab application (Figure 4a).The smartphone and CH 3 Ge 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 humid-ity response of the CH 3 Ge-decorated paper, the amount of active materials loaded on the substrate must be optimized.Here, we spray-coated three different amounts of CH 3 Ge 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 CH 3 Ge-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 CH 3 Ge (>1 mg cm −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 CH 3 Ge-decorated paper's ability to change color intensity upon various RH levels, the R/G/B values of CH 3 Ge papers were measured between RH 0% and RH 100% using a smartphone module.Figure 4d (green color square) depicts original photographs of CH 3 Ge 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 CH 3 Ge-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 CH 3 Gepaper-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 CH 3 Ge sensor in different solvent vapor.The RGB value of CH 3 Ge sensors in water vapor is the greatest, revealing high specificity in comparison to all other solvents.However, the slightly higher value observed in ethanol compared to other organic solvents is due to its >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 CH 3 Ge sensor was calculated using the increasing luminescence intensity at different RH.As shown in Figure 5b, the LOD of the CH 3 Ge-paper-based humidity sensor was as low as 9.32% RH.Furthermore, we use CH 3 Ge paper to detect ammonia gas. Figure 5c inset depicts a noticeable variation following ammonia gas exposure on CH 3 Ge, 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 CH 3 Ge intercalated by ammonia.The TD-DFT value of the optical bandgap is 2.11 eV, higher than the value for intercalated 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 CH 3 Ge 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 CH 3 Ge 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 CH 3 Ge paper.Ammonia compounds (e.g., di-and trimethylamine) are a common by-product of spoiled fish due to protein decomposition by enzymes and bacteria. [48]These results indicate that CH 3 Ge has a good response to ammonia gases and can imply the quality of foods in the beginning phases of spoilage.

CH 3 Ge-Paper-Based pH-Responsive Sensor
After successfully demonstrating the changing color intensity of CH 3 Ge in different gas environments, we found that the emission energy of CH 3 Ge 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 CH 3 Ge-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 CH 3 Ge-based pH sensor was thoroughly investigated.As shown in Figure 6a, the color of CH 3 Ge gradually changes as pH increases.When pH levels are in acidic and basic ranges, the CH 3 Ge 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 de-picts the PL spectra of CH 3 Ge 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 CH 3 Ge 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 CH 3 Ge 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 CH 3 Ge-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 increasing pH.This suggests that CH 3 Ge can be used as an emission sensor to quantitatively determine the pH of the sample, like food freshness analysis.
To assess the practical applicability of CH 3 Ge as a pH sensor, the freshness of milk was measured using the CH 3 Ge sensor and pH was quantified by a smartphone platform.The improper handling, storage, and transportation of milk causes bacteria to grow faster, posing a potential harm to consumers. [51,52]Therefore, it is essential to have a basic monitoring platform that alerts consumers about milk safety.In this a new milk freshness indicator is demonstrated that contains CH 3 Ge-decorated paper.The color change of the CH 3 Ge during milk spoilage is clearly shown in Figure 6f.The R value dropped from 164 to 130, implying a depth red color, whereas the green and blue color values changed only slightly (Figure 6e).This color change was caused primarily by the production of lactic acid, a by-product of bacterial metabolism. [53]Acidity is a key indicator for determining milk freshness.A high value of acidity denotes a greater concentration of acid components and, as a result, a lower level of freshness, that is, milk should not be consumed if the red (R) value is less than 155.

CH 3 Ge-Based Rewritable Paper for Secret Communication
Next, eco-friendly ink-free rewriteable printing paper is created by spraying CH 3 Ge flakes on a cellulose paper substrate (Figure 2a).The application of CH 3 Ge paper for water jet printing was then investigated.To better explain the reversible printing technique, a schematic illustration is presented in Figure 1c.The CH 3 Ge paper is inserted into a printer and then prepared text and images are printed by the water jet printer, causing color to appear on the CH 3 Ge paper upon illumination with 365 nm light.The printed text or images are easily removed by water evaporation heating the CH 3 Ge paper at 80 °C for 60 s and the paper can be reused.
First, different intricate patterns were printed to showcase the viability of the newly developed CH 3 Ge paper.In this investigation, an optical brightener-free paper substrate was employed.As shown in Figure 7a-g, we printed texts on the CH 3 Ge paper that were invisible to the naked eye upon ambient light.The hidden information (text) clearly appeared under exposure to 365 nm light (Movie S1, Supporting Information).After reading, the printed text can be quickly removed through a simple heating treatment and the CH 3 Ge paper can return to its initial state for the following printing session.The next two different patterns (i.e., lion and Christmas tree) were printed on the same CH 3 Ge paper with good resolution.Notably, the contrast and resolution of various printed patterns in fluorescent mode are comparable.The clear and readable characters on the CH 3 Ge paper printed by water revealed that this material can be used for high-level security printing.Further, similar patterns were printed on CH 3 Gecoated white paper (with optical brighteners) using a water jet printer.It is displaying good writability (Movie S2, Supporting Information), but the printing resolution was low due to the availability of optical brighteners and the quick water spraying feature of the paper (Figure S4, Supporting Information).Therefore, we decided to undertake all subsequent printing studies with optical brightener-free paper substrate.In general, the retaining time of water jet prints is another significant aspect that restricts the use of hydrochromic papers.Figure 7h shows the long-term stability of the printed CH 3 Ge paper.The text printed on CH 3 Ge paper via water ink can still be read well after 3 days even with some levels of fluorescence contrast reduced.
Because of the hydrochromic nature of CH 3 Ge, another potential application of CH 3 Ge paper could be for secret communications (i.e., anti-counterfeiting and information encryption).The CH 3 Ge paper can be utilized to develop images that contain secret information for the recipient that would only be visible under 365 nm light.Secret messages written on CH 3 Ge paper selferase over time or when heated, making it reusable.CH 3 Ge paper can be utilized multiple times, using water as the ink for writing, printing, and stamping.Figure 7i and Figure S5a, Supporting Information, show a quick response (QR) code and 1D code (Morse code) pattern, respectively, printed on the CH 3 Ge paper.When illuminated with 365 nm light, the writing conveys confidential information, which is then removed by heating.These findings suggest that the CH 3 Ge paper can be used in the future for information encryption and decryption.Additionally, CH 3 Ge paper can be used for stamping.Figure S5b, Supporting Information, demonstrates secret stamping (or a security mark) on the CH 3 Ge paper by dipping a pre-designed stamp in water.

Conclusion
An interesting and novel CH 3 Ge-based fluorometric sensing platform was successfully developed for gases and pH monitoring, and anti-counterfeiting applications.We found that gas molecules' intercalation into the interlayer space of CH 3 Ge results in the emission of light that can be identified with the naked eye but distinguishing the intensity change is difficult.The image captured by the smartphone camera was processed and analyzed with Color Grab color intensity processing software.To give a real-world demonstration of the CH 3 Ge sensor, real samples were used to monitor ammonia and organic acid produced by spoiled fish and milk.This study could lead to a promising method to monitor the shelf life of perishable foods, allowing consumers to receive real-time information about the quality and safety of the product.Further, DFT calculations were used to study the bandgap and optical response of CH 3 Ge altered in various gas molecules.This smartphone-assisted fluorometric sensing system gives an affordable, simple, efficient, and portable platform for tracking a variety of gases and pH levels.Moreover, we developed a simple approach for rewritable communication media based on CH 3 Ge paper.We successfully printed complex text and patterns using a water jet printer.The secret information was written on CH 3 Ge paper, which self-erases with time or by simple heating to make it reusable again.Although the primary focus of this work is to use CH 3 Ge paper for gas sensing, pH sensing, and secret communication, the use of other covalently functionalized germananes can be further applied, for example, for biosensing, fingerprints, and healthcare.

Experimental Section
Synthesis of CH 3 Ge: CH 3 Ge was synthesized using the protocol previously reported by this group. [14,23]In brief, 0.4 gm of CaGe 2 was added to methyl iodide followed by the addition of water and acetonitrile.The reac- tion mixture was then stirred at room temperature for 7 days.The obtained solid product was washed with water and acetonitrile.
Preparation of CH 3 Ge-Based Paper: To make a colloidal suspension, 5 mg of multilayer CH 3 Ge powder was sonicated for 30 min in 15 mL of absolute ethanol.Then, CH 3 Ge flakes were separated by centrifugation at 5000 rpm for 20 min.The obtained CH 3 Ge flakes were sonicated once more until they were homogeneously dispersed.Afterward, the CH 3 Ge solution was sprayed onto the cellulose paper (with and without optical brightener papers) by an airbrush (Master Airbrush Model G-22) equipped with a 0.3 mm nozzle size and 25 psi air pressure.The distance between the airbrush and cellulose paper was maintained at around 15 cm and perpendicular to one another.To achieve a uniform thin coating, the airbrush was slowly shifted in all directions.After each layer of deposition, the paper was dried using a heating plate.On cellulose paper, different amounts of CH 3 Ge coated; 0.3 mg cm −2 (sample 1), 0.65 mg cm −2 (sample 2), and 1 mg cm −2 (sample 3).
Color Intensity Measurement and Analysis: When gas molecules intercalated into the interlayer space of CH 3 Ge they immediately produced a luminescence.The intensity of CH 3 Ge as a function of gases (humidity and ammonia) was measured by a smartphone (Redmi Note 7 pro) installed with the Color Grab color intensity assessment APP.By pressing the capture button in the APP, the smartphone camera quickly determines the intensity (i.e., grayscale percentage and R/G/B value) of CH 3 Ge samples.To evaluate the fluorometric response of CH 3 Ge paper toward the pH, the bulk solution was modified drop-by-drop using NaOH and lactic acid.The pH of bulk solution was initially brought up to 7.0 by the addition of NaOH and then gradually decreased with lactic acid.Following that, 100 μL of each pH solution was placed on CH 3 Ge paper to measure luminescence intensity.In addition, photoluminescence spectroscopy was used to determine the intensity of CH 3 Ge paper in various humid conditions and pH of water.Note that all measurements were taken three times.
Food Quality Analysis: The CH 3 Ge-coated paper was used to investigate its color change properties concerning milk and fish spoilage during storage.For the fish analysis, 20 g of fish meat was stored in a plastic box with CH 3 Ge paper attached in the middle of the box lid.To avoid air transfer, the box was carefully sealed and regularly monitored using a mobile APP.For the milk analysis, 30 mL of pasteurized fresh milk was poured into a plastic bottle and left at room temperature for a week to produce lactic acid.Following that, 100 μL of the fresh and spoiled milk were placed on CH 3 Ge paper for capturing images and fluorometric analysis using a smartphone APP.All of the above experiments were repeated three times.
DFT Calculations: DFT calculations were performed using the projector-augmented wave method in the Vienna Ab initio Simulation Package (VASP). [54,55]Bulk fully functionalized CH 3 Ge in the 1T unit cell was considered as the computational model.The cut-off energy for the plane-wave expansion was set to 500 eV owing to the most recent GW PAW potentials used for Ge, O, C, and H.An optimized van der Waals functional (optB86b-vdW) was used for the calculations of structure relaxation and formation energy. [56]The formation energy was used to gauge the thermodynamics of the intercalation of water and ammonia molecules.The formation energy was calculated as where E interc. is the total energy of the bulk CH 3 Ge having intercalated given molecule, E CH 3 Ge is the total energy of bare (no intercalation) CH 3 Ge, E molecule is the total energy of a given molecule, and n is the number of intercalated molecules per unit cell.For relaxation of the bulk unit cell of CH 3 Ge, 12 × 12 × 5 k-point sampling was used.Calculations of the optical bandgap were performed using TD-DFT in connection with the hybrid functional HSE06 [57] as the exchange-correlation kernel.The calculation was performed in three steps: standard DFT calculation, hybrid DFT calculation (HSE06 functional) to obtain wavefunctions, and their derivatives used as an input for the TD-DFT calculation in the last step.The 8 × 8 × 2 k-point sampling method was used for more demanding optical calculations.

Figure 1 .
Figure 1.Smartphone-assisted fluorometric platform using CH 3 Ge-based paper for sensing and secret communication.a) CH 3 Ge-based gas sensors for humidity monitoring and food quality analysis (by ammonia sensing).b) CH 3 Ge-based pH-responsive sensor for food quality analysis (by lactic acid sensing).c) Schematic illustration of CH 3 Ge-based rewritable paper with water jet printing for anti-counterfeiting and encrypted communications.d) Color intensity was determined using a smartphone module.

Figure 2 .
Figure 2. Preparation and morphology study of CH 3 Ge.a) Synthesis procedure of CH 3 Ge and spray-coated CH 3 Ge-based paper.b,c) Low-and highresolution SEM images of bulk CH 3 Ge.d) STEM image of CH 3 Ge flakes.e,f) EDS images of CH 3 Ge flakes.g) SEM images of CH 3 Ge-coated paper.

Figure 3 .
Figure 3. Structural and photophysical properties of CH 3 Ge.a) Photograph of CH 3 Ge-decorated paper to change color in humid conditions upon  ex = 365 nm.b) PL spectra ( ex = 365 nm) of CH 3 Ge in dried and hydrated states.c) CH 3 Ge colors in CIE 1931 color space.d) XRD pattern of dried and hydrated CH 3 Ge.e) ATR-FTIR spectra of CH 3 Ge recorded at a different time in 30% humidity.f) DFT calculations of CH 3 Ge interacting with water molecules (germanane: light gray; carbon; dark gray; hydrogen: white; oxygen: red).

Figure 4 .
Figure 4. CH 3 Ge-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 CH 3 Ge 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 CH 3 Ge-based humidity indicator.

Figure 5 .
Figure 5. CH 3 Ge-based ammonia responsive sensor.a) Solvent vapor selectivity with CH 3 Ge-decorated paper.b) Low limit of detection (LOD) plot of CH 3 Ge sensor.c) R-value as a function of ammonia exposure.d) RGB values of CH 3 Ge in fresh and spoiled fish.Inset: digital images of CH 3 Ge papers in fresh fish (green square) and spoiled fish (black square).e) Schematic representation of color change of CH 3 Ge paper in relation to fish spoilage and quantification using a smartphone with color intensity processing software, Color Grab.

Figure 6 .
Figure 6.CH 3 Ge-based pH-responsive sensor.a) Images of the pH color change CH 3 Ge 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 CH 3 Ge in fresh and spoiled milk.f) Schematic representation color change of CH 3 Ge paper in relation to milk spoilage and quantification using a smartphone with Color Grab color intensity processing software.

Figure 7 .
Figure 7. Multilevel security printing on CH 3 Ge paper.a-g) Digital images during printing and erase cycles on CH 3 Ge paper.h) Retention time study of printed text on CH 3 Ge paper.i) QR code printing and erase cycles on CH 3 Ge paper.