Optical Detection of Acetone Using "Turn-Off" Fluorescent Rice Straw Based Cellulose Carbon Dots Imprinted onto Paper Dipstick for Diabetes Monitoring.

Persistent bad breath has been reported as a sign of serious diabetes health conditions. If an individual's breath has a strong odor of acetone, it may indicate high levels of ketones in the blood owing to diabetic ketoacidosis. Thus, acetone gas in the breath of patients with diabetes can be detected using the current easy-to-use fluorescent test dipstick. In another vein, rice straw waste is the most well-known solid pollutant worldwide. Thus, finding a simple technique to change rice straw into a valuable material is highly important. A straightforward and environmentally friendly approach for reprocessing rice straw as a starting material for the creation of fluorescent nitrogen-doped carbon dots (NCDs) has been established. The preparation process of NCDs was carried out via one-pot hydrothermal carbonization using NH4OH as a passivation substance. A testing strip was developed on the basis of cellulose CD nanoparticles (NPs) immobilized onto cellulose paper assay. The NCDs demonstrated a quantum yield of 23.76%. A fluorescence wavelength was detected at 443 nm upon applying an excitation wavelength of 354 nm. NCDs demonstrated remarkable selectivity for acetone gas as their fluorescence was definitely exposed to quenching by acetone as a consequence of the inner filter effect. A linear correlation was observed across the concentration range of 0.5-150 mM. To detect and measure acetone gas, the present cellulose paper strip has a "switch off" fluorescent signal. A readout limit was accomplished for an aqueous solution of acetone as low as 0.5 mM under ambient conditions. The chromogenic fluorescence of the cellulose assay responsiveness depends on the fluorescence quenching characteristic of the cellulose carbon dots in acetone. A thin fluorescent cellulose carbon dot layer was deposited onto the surface of cellulose strips by a simple impregnation process. CDs were made using NP morphology and analyzed using infrared spectroscopy and transmission electron microscopy. The carbon dot distribution on the paper strip was evaluated by scanning electron microscope and energy-dispersive X-ray analysis. The absorption and fluorescence spectral analyses were investigated. The paper sheets' mechanical qualities were also examined.

Introduction

Diabetes is a major global concern, with the number of cases expected to rise by 55% between 2013 and 2035.[1] In addition, it is estimated that half of those living with diabetes are undiagnosed.[2] In 2019, the World Health Organization recorded that about 1.6 million people died from diabetes with an increasing annual mortality among low and middle income countries.[3] There is a pressing need to develop low-cost, simple-to-use diabetic diagnostic tools that may be used at home. Solid-state gas sensors are potential methods for using breath tests to diagnose diseases. Many ambient temperature gas sensors are used for medical or environmental diagnostics and monitoring[4−7] Breath test devices are inexpensive, portable, easily distributable, noninvasive, and give rapid and direct results.[8] Diabetics can develop a critical condition called diabetic ketoacidosis where they exhale a strong fruity odor of acetone because acetone builds up in their blood as ketones are synthesized by their liver as an alternative energy supply for starving cells not capable to take up glucose.[9] Numerous studies show this correlation between acetone breath and a ketogenic metabolic state.[10−12] Diabetics not in a critical ketogenic state also exhale acetone gas, but at lower concentrations where a concentration greater than 1.8 ppm can be diagnostic of diabetes.[13] The commercially available acetone sensors have detection thresholds no lower than 50 ppm, which is unhelpful for diabetic diagnostics.[8] Recently, researchers have fabricated sensitive acetone detection sensors.[14,15] Righettoni et al. fabricated silicon-doped WO3 sensors that operate between 320 and 500 °C, which could detect acetone gas at concentrations as low as 20 ppb.[16] Semiconductor sensors can detect subppm acetone levels, but they operate at elevated temperatures and suffer reusability and humidity interference.[17−20] Modified semiconductor sensors of composites of larger diameters such as inclusion carbon-nanotubes are reported to have increased sensing by an increased absorbancy of the analyzed gas.[15,21−23] Narjinary et al. integrated MWCNT with SnO2 nanoparticles for acetone detection, which reduced moisture sensitivity.[14]

Developing easy-to-use breath sensors for clinical diagnosis has gained research interest in recent years.[24] Many volatile compounds and certain gases generated from the body, found in small concentrations (ppm) in human breath, can be probed to give an indication of ongoing metabolic bodily processes or the presence of a pathology, such as ammonia for renal failure, acetone for diabetes, H2O2 and NO for asthma, H2 for digestion disorders, and ethane, pentane, and malondialdehyde for lipid peroxidation.[25,26] Many assaying methods have been used for detecting certain pathognomic breath biomarkers including gas chromatography (GC) coupled with flame ionization detection,[27] mass spectroscopy (MS),[28] ion mobility spectrometry,[29] and proton-transfer-reaction MS.[30] However, these methods require expensive devices needing delicate handling, which limits their potential for the development of easily accessible bedside diagnostic applications. Both colorimetric and fluorescent gas sensors use a visual detection method where an immediate color change is seen by the eye when the chemical analyte contacts the receptor.[31−41] Sensors based on changes of colors have been used in various fields including environmental studies, analytical chemistry, forensics, and biological diagnostics.[42−46] Fluorescent sensors showing turn-on, turn-off, or turn-on-off behavior when in contact with the analyte were developed for the detection of different substances, such as ammonia gas, metal ions, and biothiols.[47,48] Fluorescent sensors are a composite of different materials such as dyes, fluorescent proteins, carbon dots (CDs), metal–organic frameworks (MOFs), and semiconductor quantum dots (QDs).[49] Interaction between fluorescent materials with analyte substances is either by covalent interactions or noncovalent interactions: coordination, hydrogen bonds, π–π interactions, hydrophobic- or hydrophilic-based interactions, donor/acceptor pairs, or electrostatic interactions. These interactions change the characteristics based on spectroscopy of the fluorescent material.[50]

Fluorescent carbon dot nanoparticles have the advantage of biocompatibility, simple and tunable functionality, excellent photostability, and inexpensive manufacturing.[50−52] They characterized by their broad applications including drug delivery,[53] bioimaging,[54] solar energy conversion,[55] photocatalysis,[56] optoelectronics,[57] and chemical sensors.[58] They have a benefit over semiconductors in that they are less harmful to the environment.[41] In addition, they are superior to organic fluorophores because of their high photostability and unblinking photoluminescence.[59] Carbon dots have a broad absorption spectrum that includes the UV region extending to the visible region as well as a broad florescence emission spectrum, which makes them suitable materials for fluorescence sensing of a variety of organic and inorganic substances.[60] Under a UV lamp, they showed blue fluorescence.[61] The photoluminescent stability for polymerizing carbon dots in a solid state as a nanocomposite is sometimes more stable than polymerization in solution, preserving luminescence for up to four months.[61] Carbon dots-based sensors are utilized for selective detection of pollutants,[62] metal ions,[63] biomarkers,[64] biomolecules,[65] explosives,[66] vitamins,[67] and drugs.[68] For example, a carbon dots-based fluorescent sensor is reported to monitor select ions (Cu2+, S2–) with high selectivity and sensitivity while having low cytotoxicity and high photostability in aqueous media, lasting up to 35 days.[69] Incorporating carbon dots on sol–gel fibers allow reversible optical sensing for Hg(II) through a quenching mechanism.[70] Florescent quenching dynamics may be described by the electron-transfer process, which involves electron transfer in the excited state in the carbon dots to the orbital of the quencher.[61] Borse et al. developed a dynamic carbon dots fluorescent dual mode sensor where the turn-off mode works for selective sensing of silver ion, which caused fluorescent quenching.[71] Surface modification of carbon dots with rich functional groups enhances their selectivity toward specific chemical analyte types and increases their biocompatibility.[72−74] Campos and co-workers used a carbon dots fluorescent sensor modified with surface functional groups, which enhance selectivity, to monitor organic nitro compounds.[75] Carbon dot particles are easily synthesized either by a top-down (large carbon particle precursors to smaller nanofragments) or bottom-up technique (small carbon molecule precursors to nanocluster formations).[76] Currently, carbon dots are synthesized with various procedures such as via microwaving,[69] laser irradiation,[77] pyrolysis,[78] carbonization,[79] or hydrothermal means.[80] Bottom-up hydrothermal methods to synthesize carbon dots showed high quantum yield (QY: 69%) and better resulting fluorescence properties.[81] Carbon dots have surface functional groups, such as −COOH, −OH, −OR, and −C=O, which allow binding with the analyte species.[41] The selectivity and quantum yield of carbon dots is improved by doping them with inorganic atoms, creating more functional groups, and producing a shift in their UV and Vis spectral profile.[82,83] In this work, we developed a turn-off carbon-dots based fluorescent material as a noninvasive quantization sensor of acetone, the chemical signature for diabetes. The turn-off mode is realized by quenching carbon dots’ efficiency in interactions with acetone, which can be observed by the naked eye. This easy-to-use carbon-dots based fluorescent substance was embedded in a cellulose fiber dipstick for quick qualitative and quantitative recognition of acetone gas.

In this context, detection of acetone gas for monitoring diabetes has been vital. However, the conventional detection methods have faced some disadvantages, such as complicated and costly instrumentation and sophisticated and time-consuming processing. On the other hand, the detection methods depending on fluorescence can prevent the above drawbacks and demonstrate the merits of simple manipulation. The prepared CDs have demonstrated outstanding fluorescence characteristics and presented characteristics as compared to other emissive materials due to their positive characteristics, such as biocompatibility, nontoxicity, and low cost. Herein, a simple and green strategy to prepare efficient CDs from rice straw wastes via one-pot hydrothermal technique is described. The current strategy is characterized by environmentally friendliness, low cost, and simple large-scale production. The quantum yield of carbon dots was improved by introducing a nitrogen element into the surface of carbon dots utilizing ammonium hydroxide as a passivating agent instead of utilizing any other costly and toxic amine-terminated passivating agents. The prepared NCDs can be utilized as a sensitive and selective fluorescent “on–off” sensor for detection of acetone in either aqueous or gaseous phases. The findings of the current study would not only afford ways to take advantage of rice straw waste as a valuable material but also afford cheap and green biomass toward the production of high-quality carbon dots.

Experimental Section

Materials

Acetone, amoxicillin (AMO), lysine (Lys), gentamicin (GEN), clarithromycin (CLA), tyrosine (TYR), aspartic acid (ASP), threonine (THR), tryptophan (TRY), valine (VAL), tetrabutylammonium acetate (TBAA), ammonium hydroxide (NH4OH), hydrogen peroxide (H2O2; HP), ethane (ET), pentane (PN), malondialdehyde (MDA), acetic anhydride, sodium hydroxide (NaOH), dimethyl sulfoxide (DMSO), and sodium chlorite (NaClO2), were supplied from Sigma-Aldrich, Merck, and Fluka. Distilled water was utilized for all of the experimental procedures. Wahttman (off-white) paper strips were supplied from Merck (Egypt). The paper strips were characterized by a diameter of 240 mm, thickness of 180 μm, pore size of 11 μm, and weight of 87 g/m2.

Synthesis of Cellulose Diacetate (CDA)

Rice straw waste was supplied from Egypt’s Sharqia. Rice straw was used to prepare cellulose diacetate as previously described.[84,85] The rice straw waste was repeatedly rinsed with water to get rid of dust and other solid pollutants and then chopped up and dried for 1 h at 80 °C. A 17.5% (w/w) portion of sodium hydroxide aqueous solution was used to soak the straw parts (∼5 cm) for 2 h. After rinsing and blending the swollen straws for around 30 min in a blender, the straws were air-dried under ambient circumstances. Sodium hydroxide, when introduced into the cellulosic rice straw amorphous zones, dissolves the intermolecular bonds in the cell walls of the plant. It was necessary to hydrolyze the resultant mass to separate any remaining hemicellulose and cellulose amorphous contents that were still present. Hydrolysis at 80 °C with various acid concentrations (1, 2, and 3 mol) and different durations (1, 2, and 3 h) was carried out using a constant stirrer speed. The bulk’s pH was restored to a neutral condition by exposure to deionized water followed by air-drying. The hydrolyzed bulk was washed and dried after 2 h of exposure to [NaOH](aq) (2%; w/v) at 80 °C with a constant stirring rate. It remained brown after alkali treatment because of the insoluble lignin. To finish the bleaching procedure, the sample was treated with [NaClO2](aq) (20%; w/v) at 50 °C for 60 min. The fibers’ κ number was utilized to determine the amount of NaClO2 that would be required. The fibers were cleaned and dried after being bleached. It took 30 min of homogenization at 20 kHz and 400 W in a 1.5 cm diameter cylindrical probe to fibrillate nanofibers from the chemically pure cellulose fiber slurry (1% w/w) dispersed in distilled water (125 mL). In order to regulate the temperature, homogenization was carried out in an ice bath. According to prior research.[85] CDA was generated with a DS of 2.58. The rice straw based cellulose fibers were gradually added to the DMSO–TBAA (8:2) solvent combination. After 30 min of stirring at 60 °C, a clear cellulose solution was provided (8% w/w). The admixture was then gradually infused with acetic anhydride. After 3 h of stirring at 60 °C, the solution was exposed to methanol precipitation. After filtering, washing with methanol, and drying under vacuum, CDA was obtained.

Preparation of Nitrogen-Doped Carbon Dots

According to prior techniques,[86] NCDs were made using the hydrothermal process. The produced cellulose diacetate (0.2 g) was diluted to 20 mL with deionized water after being treated with ammonium hydroxide solution. To heat the mixture at 240 °C for 15 h, we utilized a Teflon-lined hydrothermal autoclave (50 mL). The autoclave was returned to room temperature when the reaction had been completed. The admixture was centrifuged for 10 min at 10000 rpm to take away dark precipitation (byproducts). Following ultrafine filter (0.22 μm) purification, impurities were eliminated by dialysis for 24 h with deionized water in a dialysis vial (MWCO-1000). The generated NCDs was maintained at 4 °C for analyses.

Immobilization of NCDs onto Paper Strips

Dispersing various concentrations of NCDs (0, 10, 20, 30, 40, 50, 60, 70, and 80 mg) in deionized water (100 mL) yielded various solutions represented by S0, S1, S2, S3, S4, S5, S6, S7, and S8. A buffer solution of Na2HPO4 (200 μM) was then added to the above mixture. Each solution was applied directly onto a specific piece (3 × 10 cm) of unbleached Wahttman cellulose paper strip by immersion for 5 min followed by air-drying for 1 h. a Canon A710IS camera was used to photograph the NCDs imprinted sheets under a 6 W UV light set (365 nm).

Characterization Methods

The photoluminescence spectra were collected using the JASCO FP-6500 fluorescence spectrophotometer (Tokyo, Japan). The carbon dots absorbance spectra were determined by using a UV1006-M031 spectrometer (China). The FTIR spectra of NCDs and NCDs imprinted strips were collected by a Nexus 670 (Nicolet, United States). The morphological and chemical contents of NCD imprinted paper sheets were studied uisng a Quanta FEG250 SEM (Czech Republic) combined with EDX. Bruker Advance D8 (Bruker; Germany) was applied to report the XRD patterns. Zwick Universal (Germany) was used to measure the imprinted sheets’ mechanical changes with increasing the NCDs concentration on the paper sheets. An average of five measurements was reported for each sample. The form and size of NCDs were examined using TEM (JEOL1230; Japan). At an excitation of 365 nm, eq was employed to determine the QY of NCDs in an aqueous solution of H2SO4 (0.1 M) using quinine sulfate as a standard reference (QY of 54%)[87]where r is the standard sample, η is the refractive index of solvent, A is the absorbance intensity, and I is the emission intensity.

Colorimetric Properties

Before and after irradiation with UV, the color data (CIE Lab; International Commission on Illumination) and color strength (K/S) were recorded using UltraScanPro (HunterLab; United States).[88] The three-dimensional CIE Lab color coordinates were explored, including L* (100 for white; 0 for black), a* (color ratio of red to [+a*] to green to represent [−a*]), and b* (color ratio of yellow [+b*] to blue [−b*]). Samples were exposed to a UV light (365 nm) for 5 min to quantify the color of NCD-imprinted sheets. Ultraviolet light supply was placed 4 cm above the sample. The colorimetric data were promptly captured when the UV source was switched off.

Detection of Acetone

The application of NCD-imprinted paper strips to detect acetone was carried out at room temperature in both gaseous and aqueous (deionized water) phases. An aqueous solution of acetone (100 μL) was added at different concentrations (0.5–150 mM), which were then diluted to 50 mL using deionized water. The NCD-imprinted paper strip was immersed in each solution for 5 min and air-dried for 30 min. The emission spectra were then measured at 354 nm through excitation. To examine the sensor strip against gaseous acetone, pure acetone (10 mL) was charged into a glass vial (20 mL). Under ambient conditions, the sensor strip was placed near the top of the glass vial to indicate quenching of blue emission owing to exposure to gaseous acetone. In order to evaluate the sensor selectivity to acetone, the NCD-imprinted paper strip was impregnated into solutions of various compounds, including GEN, CLA, AMO, TYR, LYS, VAL, ASP, THR, Ca(II), K(I), and Na(I) under similar conditions. The tested NCD-imprinted paper strips were then analyzed.

Sensor Reversibility

The emission intensities of S6 were reported in the presence and absence of gaseous acetone to indicate no changes in the values of those emission intensities over several cycles to indicate high durability and reversibility.

Results and Discussion

Preparation of NCDs from Rice Straw

Hydrothermal treatment of cellulose diacetate results in the synthesis of NCDs as depicted in Scheme . Rice straw was steeped in [NaOH](aq) and subjected to hydrolysis by [HCl](aq). The generated fibrous cellulose was treated with a combination of TBAA/DMSO followed by treatment with acetic anhydride to give cellulose diacetate. NH4OH(aq) was utilized instead of other expensive and hazardous passivation compounds, which frequently introduce numerous terminal amines. The mass ratio of NH4OH to CDA was examined to establish the optimal QY of NCDs as illustrated in Figure . The weight of NH4OH was increased from 0 to 10 to enhance QY from 10.07 to 23.76%, respectively. The QY of NCDs would drop further when the weight of NH4OH was further raised to 15. Previous investigations on degradation/carbonization of cellulose revealed that NCDs can be generated via hydrolysis of CDA and subsequently condensation of short fragments to give NCDs. Glycoside linkages join glucose moieties in cellulose diacetate.[85] Fructose, cellulose diacetate, ethanoic acid, and cellulose diacetate intermediates can be produced in NH4OH(aq) by breakage of the glycoside bond and deacetylating cellulose diacetate.[85,86] NH4OH has been also suspected to generate forms of O- and N-bearing moieties. The indicated CDA fragments were exposed to additional condensation/carbonization to provide NCDs.[86] Ammonia nucleophilic substitution onto the carbon’s hydroxyl on the NCD surface was the key for effective passivation of nitrogen on the NCD surface,[89] which demonstrated well-distributed spherical nanoparticles. Figure shows NCDs with diameters around 40 nm as demonstrated by the TEM images. According to previous research,[89,90] NCDs have no lattice structure. There are two large peaks in the XRD spectrum of CDA at 7.5° and 20.3°. Those peaks could be attributed to the typical amorphous phase of CDA. On the other hand, the CDs only displayed two broad peaks at 15.3 and 23.1°, which are related to carbon’s amorphous phase (Figure ). NCDs surface functional substituents were also identified using FT-IR spectra (Figure ). Both NH and OH stretching vibrations were detected at 3472 cm–1, which were also associated with bending vibrations at 1244 and 716 cm–1, respectively. Those peaks indicate a successful nitrogen-doping of CDA. Both SO3 and C–O stretching vibrations detected at 1014 cm–1 proved the presence of sulfur in NCDs.[86,91]

Scheme 1

Preparation Method of NCDs

Figure 1

Effects of NH4OH/CDA ratio on the quantum yield of NCDs.

Figure 2

Transmission electron microscopic graphs of NCDs.

Figure 3

X-ray diffraction spectral analysis of CDA (a), and NCDs (b).

Figure 4

FTIR spectra of CDA, and NCDs.

Preparation of NCD-Imprinted Paper Strips

The prepared NCDs were coated onto paper sheets by simple impregnation. Through exposure to UV light, the photographs of the NCD-imprinted paper strips displayed a distinct and blue emission (Figure ). The present NCD fluorescent material has the potential to be utilized for on-site detection of acetone for real time monitoring of diabetes due to its positive properties, including low cost and simplicity of preparation and use. The coated strips’ photoswitchability to blue emission was increased by the dark off-white background of the blank paper strip.[88] The off-white paper strips exhibit a blue emission color under UV light to the naked eye better than bleached white surfaces. The UV irradiation caused all NCD-imprinted paper strips to demonstrate quick and reversible fluorescence performance.

Figure 5

NCD-imprinted paper strips showing a white color under sunlight, and blue emission under UV irradiation.

Colorimetric Screening of Paper Strips

The ability of the NCD-imprinted paper strips to alter color was studied. As can be shown in Table , the CIE Lab color screening was used to assess the technical performance of the photoswitchable NCD-imprinted paper strips. Both blank and treated strips had an off-white color to confirm that the coated film is transparent. As the NCD ratio was increased, the K/S rose in either sunlight or ultraviolet light, suggesting that the tinctorial strength had improved. As a result of rising NCDs ratios in daylight, the K/S slightly rose under sunlight, indicating the production of a weak yellow hue, and greatly increased under UV rays, indicating a deeper blue color. When the overall quantity of NCDs was increased, the L*, a*, and b* of the coated paper strips were not substantially different from those of the off-white uncoated strip in daylight. The low NCDs concentration in the coated film matrix resulted in no variation in CIE Lab and K/S values under daytime light, indicating transparency of photochromic film. A total NCD concentration of 60 mg yielded the best results in terms of colorimetric changes. When the treated samples were exposed to ultraviolet light, the excitation and emission intensities of the photoswitchable coated layer rose. Both −a* and +b* displayed minimal changes under daytime light when the NCD ratio was increased, but L* showed a modest drop with this ratio increase. Both −a* and −b* increased under ultraviolet light, and L* dropped when the coated paper was irradiated with UV light, suggesting that a colorless coating was used to cover the paper. This shows that the color of the coated paper strips changed to blue.

Table 1

Colorimetric Screening of Paper Strips Coated with Various Ratios of NCDs under Daylight (DL) and Ultraviolet Light (UVL)

	L*		a*		b*		K/S
NCDs (wt %)	DL	UVL	DL	UVL	DL	UVL	DL	UVL
S0	91.82	91.60	–3.15	–3.40	1.28	1.51	0.84	0.91
S1	90.06	85.70	–3.84	18.72	3.80	–3.61	2.49	2.89
S2	89.27	85.11	–3.19	15.28	4.86	–9.01	2.53	3.07
S3	89.20	83.65	–4.36	12.64	5.52	–9.82	2.65	3.21
S4	88.77	83.29	–2.67	9.10	6.45	–10.74	2.91	3.40
S5	88.47	82.50	–1.89	5.59	7.50	–12.39	3.10	3.88
S6	88.04	82.37	–1.15	4.32	8.02	–12.95	3.51	4.62
S7	86.68	82.02	–0.73	3.40	9.36	–14.18	3.84	4.90
S8	85.44	81.29	–0.39	2.83	10.90	–16.30	4.32	5.59

Morphological Properties of NCD-Imprinted Strips

Despite of its significance as a primary source of food throughout the globe, rice generates a lot of husks and straws every year, with a yearly output of roughly 8 × 1011 kg of straws and 1.5 × 1011 kg of husk.[92] A high quantity of rice straw is burned in the field or subjected to mulching for the following harvest. Only around 20% of rice straw is now being used for convenient purposes such as the production of animal food, paper, fertilizers, and biofuel.[93−95] In the soil, rice straw, on the other hand, is said to deteriorate over time and could be the source of rice disease. As a result of greenhouse gas emissions and smoke, burning rice straw has become more unsuitable as a disposal method for waste. Moraes et al. have studied the rice production cycle’s components.[96] Efforts have been made to develop agricultural waste uses that are both commercially and socially viable. Because rice bran has the capacity to limit the colonization of Salmonella bacteria in the gastrointestinal tracts, it has potential for functional food applications. As an oil source, rice straw has a wide range of advantages for both people and animals.[97] It has been suggested that rice straw might be used to produce biofuel and other goods.[98] Although husks have been formerly considered rubbish and often burned, a greater part of this waste was put to use since it was created off-site. But rice straw has been utilized for small-scale energy generation for a long time since it is readily available and affordable. In recent years, a range of rice straw products have been created, including polymer composite resins and lumbers, which were manufactured by combining milled straws with polymer resin.[99−101] It is possible for products with smart features to respond to environmental stimuli, resulting in unique variable properties.[102−104] An example of smart products that change color upon exposure to light includes fluorescent sensors. It has recently been more popular to develop sensor strips because of their simple preparation and use, adaptable structure, and potential applications in many fields like logic gates, medical and environmental sensors, and security alerts. The fluorescent NCD film coated onto paper strip was inspected by SEM images as depicted in Figure . Imprinting NCDs onto paper was confirmed by SEM images, which showed that a thin covering of nitrogen-doped carbon dots had been applied. The SEM images show that the deposition method was able to effectively disperse NCDs onto the cellulose sheet surface. Because of their nanoscaled nature, NCDs were evenly dispersed across the cellulose surface. Using SEM images, it was found that both coated and uncoated paper surfaces had no physical variations in their fibrous structure. As indicated in Table , EDX was employed to determine the elemental concentration of S6 at different spots on the strip surface. The elemental contents of the NCDs imprinted strips were found to be almost same at the three inspected locations. Sulfur and nitrogen levels in NCDs were found to be 1.56% and 12.16%, respectively, which are higher than those in the CDA to further confirm the effectiveness of doping. Only cellulose diacetate and ammonium hydroxide were utilized in producing NCDs, while nitrogen was mostly derived from NH4OH and the carbon source was assigned to CDA.

Figure 6

Scanning electron microscopic images of S1 (a–c) and S8 (d–f).

Table 2

Elemental Ratios of S0 and N-CD Imprinted Strip (S6) at Three Positions on the Strip Surface

strip		C	O	N	S
S0		61.82	40.18	0	0
S6	a	49.29	36.38	12.88	1.45
	b	49.18	37.10	12.16	1.56
	c	48.99	36.45	12.96	1.60

Mechanical Properties of NCD-Imprinted Strips

The paper strips coated with NCDs were tested for their mechanical features. Young’s modulus, tensile strength, and strain % of paper strips imprinted with varying amounts of NCDs content are shown in Figure . Young’s modulus and tensile strength were increased when the total content of NCDs was increased from S0 to S6, but only minor variations were seen for the concentrations from S6 to S8. As the N-CD content increased from S0 to S8, the proportion of strain at the break changed only a small amount.

Figure 7

Effects of NCDs total content on the mechanical features.

Sensing of Acetone

In the current study, a “turn-off” emission sensor for acetone was developed using an NCD-imprinted paper strip as the NCD emission can be reversibly quenched by acetone. In a pH range of 5–7, the NCD-imprinted paper strip (S6) demonstrated high fluorescence stability. The efficiency of quenching (F/F0) was determined depending on the fluorescence of the NCD-imprinted paper strip in the presence and absence of acetone, which was represented by F0 and F, respectively. The emission spectral analysis and efficiency of quenching upon immersion of the NCD-imprinted paper strip in solutions of acetone at different concentrations are depicted in Figure . When the acetone content was increased from 0.5 to 150 mM, the maximum fluorescence intensity of the NCD-imprinted paper strip progressively dropped. A linear relation was observed between the efficiency of quenching and the acetone concentrations between 0.5 and 150 mM. In addition, the limit of detection was determined to be 0.5 mM depending on the rule of 3σ. The current NCD-imprinted paper strip demonstrated a broader linear range (0.5–150 mM) and comparatively low detection limit (0.5 mM). The findings proved that the precision of the current NCD-imprinted paper strip was close to or better than that of previously reported techniques. To study the acetone detection selectively for the current NCD-imprinted paper strip, other compounds might exist in the physiological systems, such as antibiotics (AMO, CLA, GEN), amino acids (TYR, LYS, VAL, ASP, THR), metal ions [Ca(II), K(1), Na(I)], and others (ET, PN, HP, MDA). These compounds were also tested, and their emission spectra and quenching efficiency (Figure ) were recorded under the same conditions employed in the detection of acetone. The emission maxima of NCD-imprinted paper strips were found to barely change upon immersion in solutions of the above competitive compounds, which was useful for estimating the quenching efficiency. The acetone solution displayed an apparent quenching of fluorescence with a high decrease of ∼85% intensity. On the other hand, the NCD-imprinted paper strip showed a weaker responsiveness to the majority of other compounds to indicate a remarkable selectivity to acetone. The potential emission quenching mechanism of NCDs was explored. The aggregation of NCDs upon addition of acetone was assessed by TEM as shown in Figure . It was shown that no bigger particles of NCDs were produced upon immersion in the acetone solution. The average diameters of NCD particles after immersion in the acetone solution were detected at 1.76 nm, which is similar to the average diameters of the NCD particle before (1.79 nm) immersion in the acetone solution. The quenching of the NCD emission was most likely attributed to the static adsorption.[86] The excitation spectral analysis of the NCD aqueous solution was collected as illustrated in Figure . The absorbance spectrum of acetone was found to overlap well with the excitation spectrum of NCDs, while only portions of it were found to overlap with the fluorescence spectrum of NCDs. These results indicate that the quenching of emission could be attributed to an IFE effect, which is known as the fluorescence resonance energy transfer (FRET).[86] To prove the IFE effect, the emission lifetimes of the NCD-imprinted paper strip before and after exposure of acetone were determined and reported to be 7.47 and 7.25 ns, respectively. The emission lifetime of the NCD-imprinted paper strip before and after exposure of acetone was barely varied to confirm the nonexistence of FRET among NCDs and acetone. Figure displays the decay times of the NCD-imprinted paper strip before and after exposure of acetone.

Figure 8

Fluorescence spectra of NCD-imprinted paper strip (S6) after exposure to acetone at different contents (0.5–150 mM); F/F0 as a function of acetone concentration is a linear correlation in the range of 0.5–150 mM: 0.5 (a), 25 (b), 50 (c), 70 (d), 90 (e), 110 (f), 130 (g), 150 (h) mM.

Figure 9

Selectivity of NCD-imprinted paper strip to acetone; emission spectra of NCD-imprinted paper strip in the existence of antibiotics, amino acids, and metallic cations at a fixed concentration of 100 μM.

Figure 10

Excitation spectrum of NCD-imprinted paper strip S6 (top) and NCDs in aqueous solution under visible and UV lights (bottom).

Figure 11

Decay time of NCD-imprinted paper strip before (a) and after (b) exposure of acetone.

Reversibility and Durability

Figure displays the testing results of the NCDs imprinted strip for fatigue resistance upon exposure to acetone and ultraviolet irradiation. The fluorescent strip (S6) was exposed to gaseous acetone and then taken away from the acetone source. The emission intensity was reported for every cycle of exposure to acetone. No changes were detected in the emission intensities in the presence and absence of acetone gas over several cycles to indicate high durability and reversibility. Additionally, the fluorescent strip (S6) demonstrated good durability as the same fluorescence intensity was detected without changes after 3 weeks of storage.

Figure 12

Reversibility of fluorescence at 443 nm (S6).

Conclusion

In summary, valuable nanomaterials were developed from rice straw waste for high technical applications. In a one-pot hydrothermal reaction toward the production of NCDs from cellulose diacetate generated from rice straw waste, NH4OH was utilized as a passivation substance, which is a cheap and harmless method. It was also investigated how NCDs may be formed. NCDs have outstanding emission with a QY of up to 23.76%. SEM, EDX, XRF, fluorescence spectra, and colorimetric screening were all used to examine the luminous security films deposited onto paper strips. Under daylight, the printed security films seemed to have an off-white appearance. Photochromic blue emission color detected under ultraviolet light was found to be quenched upon exposure to acetone. The mechanical characteristics of the NCD-imprinted paper strip were not significantly different from those of the blank paper sheets. The luminous layer’s transparency was achieved by a homogeneous dispersion of nanoscale CDs. The fluorescence of the NCD-imprinted paper strip was found to be selectively quenched by acetone. A linear correlation was detected between the efficiency of emission quenching and the concentration of acetone in the range of 0.5–150 mM considered as the limit of detection for 0.5 mM. The mechanism of fluorescence quenching was ascribed to the IFE effect. The current strategy not only introduces an efficient and simple method to convert rice straw waste into valuable nanosized materials but also provides an approach to prepare a viable sensor assay for determination of acetone and monitoring of diabetes.