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Silver nanoparticles-embedded nanopaper as a colorimetric chiral sensing platform
Author’s Accepted Manuscript Silver nanoparticles-embedded nanopaper as a colorimetric chiral sensing platform Erhan Zor
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S0039-9140(18)30211-X https://doi.org/10.1016/j.talanta.2018.02.096 TAL18415
To appear in: Talanta Received date: 20 January 2018 Revised date: 20 February 2018 Accepted date: 24 February 2018 Cite this article as: Erhan Zor, Silver nanoparticles-embedded nanopaper as a colorimetric chiral sensing platform, Talanta, https://doi.org/10.1016/j.talanta.2018.02.096 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Silver nanoparticles-embedded nanopaper as a colorimetric chiral sensing platform
Erhan Zor*
*Department of Science Education, A. K. Education Faculty, Necmettin Erbakan University, Konya, 42090, Turkey
*
Corresponding author:
Erhan Zor -E-mail: [email protected] and [email protected]. Tel: +90 332 323 8220/5566 Declarations of interest: none
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Abstract
Paper-based sensors open new avenue to produce simple, rapid, inexpensive and single-use analytical devices for a wide application fields involving medical diagnosis, food analysis and environmental monitoring. In this study, a new optical sensing platform for enantioselective recognition of chiral analytes was introduced by using in-situ synthesized silver nanoparticlesembedded transparent nanopaper. To this aim, nanopaper was obtained by environmentfriendly approach using bacterial cellulose made of nanofibers and silver nanoparticles were embedded within nanopaper by an in-situ generation method. The chiral recognition capability of AgNPs was operated in both solution phase and nanopaper for the tested enantiomers. AgNPs showed a discriminative sensing response toward D-cysteine with a LOD value of 4.88 µM. The principle of optical sensing is the selective interaction of the inherently chiral AgNPs with enantiomers causing to aggregation of AgNPs to display a significant colour change from yellow to purple-brown in both aqueous phase and nanopaper. As for practical use, the obtained plasmonic nanopaper was punched into circular pieces and put on wax-printed PET film to produce disposable two-dimensional cuvette which could be inserted in an ordinary spectrophotometer. The enantiomeric percentage of D-cysteine was successfully determined by the fabricated nanopaper-based cuvettes.
Graphical Abstract
2
Keywords: Chiral recognition, silver nanoparticles, nanopaper, paper-based sensor, colorimetric sensor.
1. Introduction Chirality is a global phenomenon of nature and is a major research field of chemical biology [1]. The majority of bioactive species exhibit chiral feature and many key biological mechanisms possess specific interactions of chiral molecules [2–4]. The detection of an enantiomer of chiral species can be a precious indicator for the analysis of food quality. For instance, the presence of D-amino acid in natural products can be considered as an indicator 3
for bacterial contamination. Similarly, the content of D- or L-enantiomers of lactic acid in samples manifests the type of bacteria [5]. In addition, most modern drugs and pharmaceuticals are composed of individual enantiomers of chiral species, which may exhibit higher desired effects with lower toxicities than do their antipodes [6,7]. Accordingly, the development of efficient approaches for selective screening of enantiomers of a compound and for analysis of the enantiomeric content of samples is of great interest [8–14]. Currently, the approaches which rely on chromatography techniques and electrochemistry are successfully operated for screening of the enantiomeric compositions of chiral species. However, the breakthrough techniques are still in demand to overcome the sophisticated instrumentation and develop fast and simple optical discrimination of enantiomers by visual detection. Within this respect, optical methodologies, which enable not only rapid analysis of the purities of enantiomeric species but also possess advantageous features such as ease of production, low-cost, high sensitivity and adaptation, have drawn substantial interests of researchers [15]. The key requirement of optical approaches to assess enantiomeric contents is chiral sensors that capable of differentially interact with antipodes of a chiral analyte in a manner that results in different optical responses. With the recent major developments in molecular sensing and supramolecular chemistry, varieties of enantioselective optical sensors such as small molecule, metal complex and polymer-based enantioselective sensors have been fabricated over the past decades [15]. With the recent advances in nanotechnology, nanomaterials have become an alternative and ideal constituents for the construction of optical sensing systems [16]. Noble metal nanomaterials, particularly silver nanoparticles (AgNPs) and gold nanoparticles (AuNPs), are extensively used as substances to build up an optical sensor owing to their size and shape depending optical properties [17]. In recent years, the researchers have devoted considerable eff ort to the production and application of AgNPs and AuNPs for the field of
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optical chiral discrimination due to their inherent chirality leading to enantioselective interaction towards antipodes of chiral species [18–25]. In the last decade, nanomaterial-incorporated paper-based sensors have attracted marvellous research and commercial attention due to their applicability in design of various disposable instruments for consumers [26–31]. Additionally, these kind of optical sensing platforms can easily adapt the visual detection to simple optical/electrical readers, thus no complicated devices are required [25]. As an optical sensing platform, nanopaper defined as a sheet made of cellulose nanofibers (below 50 nm) has recently been a new focus in paperbased sensors due to its unique advantages and numerous desirable properties such as high optical transparency and mechanical properties, flexibility, environmental sustainability and biodegradability [32,33]. The transparent nature of the nanopaper also makes it a very suitable substrate for incorporation or immobilization of optical sensing agents to be utilized in optical sensors [34]. Considering the
aforementioned
content,
the
silver
nanoparticles-embedded
transparent nanopaper was proposed as an optical chiral sensing platform in this study. Initially, the chiral recognition capability of silver nanoparticles was examined in solution phase. Then, the advantages of nanopaper and AgNPs were combined for the development of a disposable nanopaper-based sensor. To this aim, nanopaper was obtained by an environment-friendly approach using bacterial cellulose made of nanofibers. Silver nanoparticles were in-situ decorated within nanopaper as a result of direct reduction of silver ions by hydroxyl groups of nanofibers, without any external derivatization and reducing agents. After the operational principle of this nanopaper-based assay and its proficiency to be utilized in such an significant area were assessed, single use, robust and cost-effective waxprinted two dimensional (2D) cuvettes were developed for practical applications. By operating wax-printed cuvettes, not only to minimize utilization of the sample without any
5
exposure but also decrease the sample volume (microliters) was succeeded and made it more convenient for small volume sample analysis in an ordinary spectrophotometer. In this study, the colorimetric discrimination of enantiomers was examined and a satisfactory response was obtained for cysteine enantiomers. In the literature, several papers were reported on the cysteine (no defined D-/L- cysteine, or L-cysteine) detection using the method of aggregation of Ag or Au nanoparticles [35–41]. Differently from these studies, herein, discriminative sensing of D-/L-cysteine enantiomers was proposed by the use of AgNPs-embedded transparent nanopaper as a novel sensing strategy without the need of advanced instruments.
2. Material and methods 2.1. Reagent and apparatus Silver nitrate (AgNO3), trisodium citrate (Na3C6H5O7), NaBH4, D- and L-enantiomers of the tested chiral species and glucose, (NH4)2SO4, KH2PO4, MgSO4.7H2O, NaOH, H2O2 were purchased from Sigma-Aldrich. A JEOL JEM 2100 UHR-TEM was used for Transmission Electron Microscopy (TEM) images. Scanning electron microscopy (SEM) was performed by a ZEISS EVO LS 10 SEM. A Shimadzu UV-1800 double beam spectrophotometer was employed for UV-Vis absorption measurements. A ColorQube 8580 wax printer was used to fabricate the disposable cuvettes. The photographs were taken with a Canon EOS 700D digital camera. A Milli-Q system (Millipore) was used to obtain ultrapure water.
2.2. Fabrication of Bacterial Cellulose Nanopaper
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Bacterial cellulose nanopaper was produced by an environment-friendly method [32]. Briefly, Acetobacter xylinum bacteria culture was incubated in 1 liter of water containing glucose (50 g), yeast extract (5 g), (NH4)2SO4 (5 g), KH2PO4 (4 g) and MgSO4.7H2O (0.1 g) at 28 ˚C for two weeks. Then, the resultant (3-4 mm-thick unpurified wet and gel-like mat made of cellulose nanofibers) was first treated with 5 wt% NaOH for 24 h at ambient conditions and then with 1wt% NaOH/0.2 wt% H2O2 for 1 h at 80 ᵒC in order to purify from bacteria and the other impurities. As the second purification step, the produced wet mat made of bacterial cellulose nanofibers was washed with ultrapure water to remove the chemicals and to obtain a transparent nanopaper (Fig. S1). 2.3. Preparation of the silver nanoparticles (AgNPs) All materials used in the preparation process were cleaned with aqua regia (HNO3/HCl= 1:3, v/v) and washed with ultrapure water. Silver nanoparticles were obtained using AgNO3, and NaBH4 and trisodium citrate (TSC). The synthesis process was operated in the dark conditions to prevent photodecomposition of AgNO3. Briefly, a 62,5 μL of 0,1M AgNO3 was added into 25 mL of pure water. A 62,5 μL of 0,1M TSC was added to the resultant solution and mixed for 30 min under constant stirring at ambient temperature. Then, 1.5 mL of 0.005 M NaBH4 was incubated to the solution and the color immediately changed to light yellow and finally to amber expressing the formation of silver nanoparticles [42]. The resultant was vigorously stirred for 2h. The all procedure for the synthesis of AgNPs was shown in Fig. S2a. 2.4. Fabrication of in-Situ Generated AgNPs-embedded Nanopaper The procedure for fabrication of AgNPs-embedded nanopaper was performed by following previous literature [34]. Briefly: A 62,5 μL of 0,1M TSC was added to 20 mL of ultra pure water (pH 8.0) in a glass bottle. A piece (1 cm × 2 cm) of as-prepared wet bacterial cellulose nanopaper was added. Then, the resultant was heated (50 °C) and stirred for 15 min. 7
Then, a 5 mL of AgNO3 (0.01 M) was syringed dropwise to mixture under the ambient conditions with vigorous stirring. Afterwards, pH value of the resultant was arranged to 12.0 and it was continuously stirred at 50 °C for 2 h. The preparation of AgNPs-embedded nanopaper was preliminarily assessed with gradually change in colour from transparent to yellow-amber expressing the formation of silver nanoparticles (Fig. S2b). Afterwards, the resultant was cooled to ambient temperature. AgNPs-embedded nanopaper was separated from the solution and washed with ultrapure water to remove the precursors from the fabricated nanopaper surface. The nanopaper was punched into the circular pieces for practical use (Fig. S2c). Finally, they were stored in a closed vessel in refrigerator. It should be remarked that AgNPs-embedded nanopapers remain undestroyed for more than 12 weeks. The fabrication of AgNPs-embedded nanopaper was confirmed not only by UV-visible absorption spectra and also by SEM, EDX and elemental mapping analysis. Fig. S3 shows the EDX and elemental mapping analysis results. It is clearly seen that AgNPs-embedded nanopaper shows homogenously distributed AgNPs in elemental mapping at the selected area of the corresponding SEM image with Ag peak beside C and O peaks in EDX data indicating the presence of AgNPs over the blank nanopaper. 3. Results and discussion 3.1. The Enantioselective Response Towards Cysteine Enantiomers Noble metal nanoparticles have been reported to offer potential for chiral sensing applications due to their plasmonic properties. As initial demonstration of the potential of AgNPs for optical enantioselective sensing use, the classical colorimetric experiments have been performed in solution depending on a colour change which is a simple but efficient way fairly investigated in the literature. This operational principle has been successfully performed in various works. To illustrate, Zhang and Ye have proposed a surface modified-Ag nanoparticles (AgNPs) for visual detection of D- and L-cysteine [24]. Liu et al. have reported 8
a simple and convenient procedure for chiral recognition of aromatic amino acid with β-CD coated AgNPs by taking advantage of favorable features of β-CD (surface coating agent) and the distance dependent optical property of AgNPs (colorimetric probe) [19]. Apart from the surface-functionalized forms of AgNPs, Liu et al. have demonstrated that the citrate-capped AgNPs can be utilized as optical probe in chiral recognition of tryptophan enantiomers by using advantage of the inherently chiral AgNPs [18]. Within this respect, the overall optical enantioselective performance of unfunctionalized AgNPs was explored. To this aim, the solution including excess quantity of the used chiral species was mixed with as-prepared AgNPs solution. The change in colour was examined by naked-eye. As shown in Fig. 1a, a distinct optical change from yellow to purple with the addition of D-cysteine. Interestingly, no distinct difference was obtained in colour for L-cysteine, which indicates that AgNPs displayed enantioselective interaction towards one enantiomer of cysteine. A centrifugation process was applied to the mixed solutions of enantiomers and AgNPs. It was observed that a large quantity of precipitate occurred for the existence of D-cysteine, which was ascribed to the aggregation of AgNPs (see Fig. 1a'). Besides these observations, both enantiomers of the rest of the used chiral species have displayed either a significant or no optical response. This result indicates that both antipodes act in the same manner leading to no selective enantiorecognition process (Fig. S4). It can be ascribed to (i) both antipodes conveniently interact with AgNPs, or not; (ii) the dimentional position of the interaction and steric hindrance effecting the internanoparticle distance of AgNPs. Herein, the elucidation of enantioselective recognition mechanism for the occurring process is the major point and it is also rational to note that this AgNPs-based optical sensor is efficient for enantioselective detection of cysteine enantiomers. Within this respect, in the previously proposed chiral sensing assays involving AuNPs and AgNPs [18–25], the major element has been attributed to the enantioselective groups attached to the nanoparticles surface. On the other hand, metal
9
surfaces [43], AuNPs and AgNPs are inherently chiral species [17,44,45]. Considering the aforementioned reasons above, associated with our study, Zhang et al have developed a chiral sensor for cysteine enantiomers in aqueous sample by AgNPs modified with uridine 5´triphosphate (UTP) as enantioselective unit. In our method, the chiral recognition can be explained by inherent chirality arisen through the kink sites in the framework of metal nanoparticle (inherently chiral AgNPs) [43]. While citrate molecule does not exhibit chirality, the backbone of the citrate-capped AgNPs can endow a convenient enantioselective interaction and/or chemistry [18,24]. Moreover, convenient dimensional interaction of cysteine-adsorbed AgNPs leads to aggregation upon interaction resulting to the purplecoloured solution. Finally, the possible mechanism can be elucidated by the interaction of Dcysteine molecules which may interact with AgNPs by means of thiol (-SH) group or amino group (−NH2) with larger adsorption energy than L-cysteine molecules. After that, the interaction of at least two D-cysteine molecules can occur between their spatially neighboring carboxylic groups (−COOH) at the chiral centers leading to the most suitable dimensional interaction via hydrogen bonds. This defeats the electrostatic and steric hindrance, and finally reduce the distance between AgNPs to accumulate. As a consequence, the aforementioned results indicate that AgNPs exhibit good enantioselectivity towards D-cysteine.
Here Figure 1
Aiming at investigating the effect of pH values on enantioselective determination of cysteine enantiomers, the behavior of AgNPs was examined for pH range of 3.0-8.0. As indicated in the previous literature [21,46], AgNPs were not stable and tend to aggregate at extreme acidic and alkaline pH. In our system, AgNPs aggregated even without cysteine at
10
pH < 5.0 and pH > 8.0, and the optimum pH value was obtained to be pH 6.0 for stable AgNPs and enantiorecognition of cysteine enantiomers. As the second demonstration of enantioselective recognition of cysteine enantiomers, AgNPs solution was utilized as a colorimetric probe and the changes were followed by UVVis spectrophotometry technique in the absence and presence of cysteine enantiomers. Fig. 1b shows UV-Vis spectra of blank AgNPs solution, D-cysteine/AgNPs and L-cysteine/AgNPs solutions. A maximum absorption peak was obtained at 408 nm arising from the surface plasmon absorption of AgNPs [47]. It was red-shifted by addition of D-cysteine while only a small decrease was monitored for L-cysteine, which remarks that D-cysteine could enantioselectively bounds to AgNPs, which leads to aggregation of AgNPs, but L-cysteine displays no significant effect, as demonstrated by the colorimetric experimental results. Aiming at examining the analytical applicability of AgNPs as colorimetric probes for the detection of chiral species, the effect of cysteine enantiomers concentration on the absorption profile was assessed. Fig. 1c and 1d shows the absorption spectra of AgNPs solution displaying optical response for D- and L-cysteine. The peak at 408 nm decreased accompanied with a gradually increment of peak at 600 nm for increasing concentrations. Different from this observation for D-cysteine, no significant change on the AgNPs was observed as for increasing concentration of L-cysteine.
3.2. Application of AgNPs-embedded nanopaper for colorimetric chiral sensing Considering the results of colorimetric and spectrophotometric experiments, the applicability of a single use paper-based chiral assay using AgNPs-embedded nanopaper for practical applications have been foreseen. To assess the potential of functionalized plasmonic nanopaper for optical sensing use, AgNPs-embedded nanopaper was utilized as a colorimetric enantioselective sensor for cysteine enantiomers. Fig. 2a illustrates the change in color of
11
AgNPs-embedded nanopaper from yellow to purple-brown upon addition of different amount of D-cysteine, whereas no significant change was observed in color for addition of L-cysteine. After 100 µM D-cysteine, no change was obtained in color indicating the saturation of the sensing response.
Here Figure 2
Scheme 1 shows the overall experimental procedure and the operational concept of the developed nanopaper-based sensor. The hydrophilic character of nanopaper allows the adsorption of solution and the inherent chirality of AgNPs allows the enantioselective interaction with the enantiomers. Once the sampling of D-cysteine solution (50 µL) onto AgNPs-embedded nanopaper, the color changes through aggregation-induced by interaction of D-cysteine molecules with each other.
Here Scheme 1
For quantitative evaluation, the colour density is converted to the colour intensity (CI) percentage scale (defined as the analytical signal) by using ImageJ software. Fig. 2b displays the linear increase in colour intensity, CI, after adding different concentrations of D-cysteine (as C) between 5 µM and 100 µM. As experimental evidences, the performance of assay linearly changes (R2 = 0.993) which can be estimated by Equation 1: CI = 0.48 (±0.04) C (D-cysteine) + 12.01 (±0.71) (n=3)
(1)
12
where n is the number of performed sample. Limit of detection (LOD) value was calculated to be 4.88 µM for D-cysteine. LOD could not be estimated for L-cysteine because no distinct change was obtained at CI in the presence of different concentration of L-cysteine. As observed in the colorimetric experiments, D-cysteine can be selectively adsorbed onto AgNPs and it can act as a linker on AgNPs, thereby promoting their aggregation. This phenomenon can be seen in the TEM images in Fig. 3a-a', 3b-b' and 3c-c'. As shown in Fig. 3a-a', citrate-capped AgNPs (18 ±2 nm, spherical-shaped) were homogenously dispersed in the solution without D-cysteine or L-cysteine. Correspondingly, L-cysteine addition (Fig. 3bb') could not induced a significant aggregation of AgNPs, as evidenced in the aforementioned solution phase experiments. However, large aggregates appeared after addition of D-cysteine that could be expressed by bridge-like role of D-cysteine enantiomers (on the surfaces of AgNPs) drawing the distance between AgNPs by interaction aforementioned above and therefore giving rise to remarkable aggregation as shown in Fig. 3c-c'. Differently from the reported papers which demonstrated that several metal ions would be utilized as cross-linking agents to induce the agglomeration of nanoparticles [23,48], herein it was demonstrated that the aggregation could be directly achieved in the absence of any cross-linkers which would render the process more complicated by increasing uncertainty to understand the entire process.
Here Figure 3 The same phenomenon was observed not only in the solution phase but also on nanopaper. Fig. 3d-d', 3e-e' and 3f-f' show SEM micrographs, which respectively correspond to the prepared nanopaper and AgNPs-embedded nanopaper after adding D-cysteine and Lcysteine at low and high magnifications. The diameter of nanofibers were measured on SEM micrographs and was observed as 40 ± 10 nm. Fig. 3e-f show how D-cysteine and L-cysteine
13
affect AgNPs in nanopaper at the nanoscale. On the basis of our findings, it can be concluded that D-cysteine facilitate the aggregation of AgNPs in nanopaper, as well.
3.3. The performance of AgNPs-Embedded Nanopaper in Determination of Enantiomeric Percentage One of the essential issues in chiral detection is to examine enantiomeric percentage by the proposed platform, because enantiomers of a chiral molecule generally coexist in an environment. For this purpose, utilizing the developed assay, AgNPs-embedded nanopaper could be directly employed for determination of the percentage of cysteine in a racemic solution. As observed in Fig. S5a-b, the aggregation of AgNPs was induced by D-cysteine, and a change in color intensity was observed for different enantiomeric percentages of Dcysteine between 5% and 100% for the optimized sensor. Another interesting point to be mentioned was whether the selective sensing performance of AgNPs-embedded nanopaper would be investigated in a common spectrophotometer by using a disposable microcuvette because the AgNPs-embedded nanopaper exhibited high transparency behavior before and after sampling for enantiomeric percentage of D-cysteine in a mixed solution (see Fig. S5c). In our previous paper, we have developed a sensing system based on AuNPs-employed assay consisting of a conjugation membrane capturing the target and the detection membrane signaling the existence of the captured target [25]. The proposed paper-based device was a convenient platform for chiral sensing applications, while it possessed some drawbacks such as non-transparency of the used cellulose acetate membranes and the requirement of high volume of sample ( at least 2 mL). In this study, as for decrease the sample volume for the use in small volume of samples, the microcuvette modulation for spectroscopic measurements at solid platform was performed by using transparent AgNPs-embedded nanopaper with a very small volume of sample (50 µL) for the same percentage range of cysteine enantiomers, see
14
Fig. 4a. Our initial design criteria for the microcuvettes was the size and optically-inactive disposable platform, and thus control the overall absorption upon interaction of AgNPs and D-cysteine. Here Figure 4
As aforementioned, wax printed PET film was used as holders which were opticallyinactive and could be easily attached onto holder of spectrophotometer with a stabilized position. This preparation and easy modulation procedure of sensing assay was shown step by step in Fig. S6. In our sensing system, because of the requirement of entirely wetting of the nanopaper, an adjusted volume of sample (50 μL) was required to assure efficiently capturing of analyte which led to absorption change in confined aqueous solution by nanopaper. After this process, the treated nanopaper was finally placed onto the microcuvette to perform spectrophotometric measurements using an ordinary spectrophotometer. Fig. 4b reveals the UV-Vis spectra of microcuvettes consisting of AgNPs-embedded nanopapers for the enantiomeric percentage of D-cysteine within the operational range between 5% and 100%. As clearly seen, similar to the solution phase experimental results, an absorption peak was obtained at 408 nm arising from the surface plasmon absorption of AgNPs. The absorption peak was red-shifted (to 600 nm) at different enantiomeric percentage of D-cysteine which demonstrated that the developed microcuvette platform was a simple and rapid approach for sensitive and selective optical sensing of enantiomeric percentage of D-/L-cysteine. 4. Conclusion A simple, versatile and disposable platform has been developed for enantiosensing applications of chiral molecules by exploiting the impressive properties of nanopaper and AgNPs. Qualitative analyses have been performed by simply treating AgNPs with cysteine enantiomers and observing (with the naked eye) the generation of a change in colour of 15
AgNPs. By taking advantage of the inherently chiral yellow-coloured AgNPs that displays a colour change to purple-brown in the existence of D-cysteine, enantioselective recognition of cysteine enantiomers has been successfully achieved in solution. Also, it has been shown that this
enantioselective
platform
can
be
easily
adapted
to
useful
devices
(e.g.,
spectrophotometer) for individual assays by using simple punch tools and wax printing for holder preparation. By using this feature, quantitative measurements have been taken to determine enantiomeric percentage of cysteine enantiomers by using an ordinary spectrophotometer. Overall, this class of platforms can opens up innovative capabilities to design not only novel enantiosensing samples analysis strategies but also diagnostics, environmental monitoring and food safety systems, which can be readily analyzed by a simple software controlled via a camera supported devices, such as mobile phone or tablet computer. Appendix A. Supplementary material Supplementary data associated with this article can be found in the online version.
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Scheme Legend Scheme 1. Schematic representation of the experimental procedure and operational concept of nanopaper-based platform (not to scale) as enantioselective optical assay based on aggregation-induced by sampling of D-cysteine. 20
Figure Legends Fig. 1. The colour change of AgNPs solution with and without 100 μM of D- or L-cysteine (a). The centrifuged AgNPs solutions including of D- or L-cysteine (a'). UV-Vis spectra of as-synthesized AgNPs, with and without 100 μM of D- or L-cysteine (b). UV–Vis titration spectra for D-cysteine (c) and L-cysteine (d) between 0 and 100 µM. Fig. 2. The nanopapers before and after sampling D- and L-cysteine enantiomers between 5 and 100 µM (a). The change of colour intensity upon the presence of D- and L-cysteine between 5 and 100 µM for the optimized assay (b). Fig. 3. TEM images of blank AgNPs (a-a') (at different resolutions), with L-cysteine (b-b'), and D-cysteine (c-c') enantiomers. SEM images of nanopaper at different magnifications at 1 µm (d) and at 200 nm (d'), respectively. SEM images of AgNPs embedded-nanopaper in the presence of L-cysteine (e-e'), and D-cysteine (f-f') enantiomers at low (1 µm) and high (200 nm) magnification, respectively. Fig. 4. The microcuvette modulation for spectrophotometric measurements at solid platform for the same enantiomeric percentage range of D-cysteine (a). The recorded UV-Vis spectra for the corresponding microcuvettes (b).
Scheme 1
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Figure 1
22
Figure 2
23
Figure 3
24
Figure 4
25
Highlights
AgNPs-embedded nanopaper is developed for enantioselective sensing of chiral molecules.
Colorimetric discriminative sensing of cysteine enantiomers is achieved.
The enantiomeric percentage can be determined by the proposed sensor.
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PII: DOI: Reference:
S0039-9140(18)30211-X https://doi.org/10.1016/j.talanta.2018.02.096 TAL18415
To appear in: Talanta Received date: 20 January 2018 Revised date: 20 February 2018 Accepted date: 24 February 2018 Cite this article as: Erhan Zor, Silver nanoparticles-embedded nanopaper as a colorimetric chiral sensing platform, Talanta, https://doi.org/10.1016/j.talanta.2018.02.096 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Silver nanoparticles-embedded nanopaper as a colorimetric chiral sensing platform
Erhan Zor*
*Department of Science Education, A. K. Education Faculty, Necmettin Erbakan University, Konya, 42090, Turkey
*
Corresponding author:
Erhan Zor -E-mail: [email protected] and [email protected]. Tel: +90 332 323 8220/5566 Declarations of interest: none
1
Abstract
Paper-based sensors open new avenue to produce simple, rapid, inexpensive and single-use analytical devices for a wide application fields involving medical diagnosis, food analysis and environmental monitoring. In this study, a new optical sensing platform for enantioselective recognition of chiral analytes was introduced by using in-situ synthesized silver nanoparticlesembedded transparent nanopaper. To this aim, nanopaper was obtained by environmentfriendly approach using bacterial cellulose made of nanofibers and silver nanoparticles were embedded within nanopaper by an in-situ generation method. The chiral recognition capability of AgNPs was operated in both solution phase and nanopaper for the tested enantiomers. AgNPs showed a discriminative sensing response toward D-cysteine with a LOD value of 4.88 µM. The principle of optical sensing is the selective interaction of the inherently chiral AgNPs with enantiomers causing to aggregation of AgNPs to display a significant colour change from yellow to purple-brown in both aqueous phase and nanopaper. As for practical use, the obtained plasmonic nanopaper was punched into circular pieces and put on wax-printed PET film to produce disposable two-dimensional cuvette which could be inserted in an ordinary spectrophotometer. The enantiomeric percentage of D-cysteine was successfully determined by the fabricated nanopaper-based cuvettes.
Graphical Abstract
2
Keywords: Chiral recognition, silver nanoparticles, nanopaper, paper-based sensor, colorimetric sensor.
1. Introduction Chirality is a global phenomenon of nature and is a major research field of chemical biology [1]. The majority of bioactive species exhibit chiral feature and many key biological mechanisms possess specific interactions of chiral molecules [2–4]. The detection of an enantiomer of chiral species can be a precious indicator for the analysis of food quality. For instance, the presence of D-amino acid in natural products can be considered as an indicator 3
for bacterial contamination. Similarly, the content of D- or L-enantiomers of lactic acid in samples manifests the type of bacteria [5]. In addition, most modern drugs and pharmaceuticals are composed of individual enantiomers of chiral species, which may exhibit higher desired effects with lower toxicities than do their antipodes [6,7]. Accordingly, the development of efficient approaches for selective screening of enantiomers of a compound and for analysis of the enantiomeric content of samples is of great interest [8–14]. Currently, the approaches which rely on chromatography techniques and electrochemistry are successfully operated for screening of the enantiomeric compositions of chiral species. However, the breakthrough techniques are still in demand to overcome the sophisticated instrumentation and develop fast and simple optical discrimination of enantiomers by visual detection. Within this respect, optical methodologies, which enable not only rapid analysis of the purities of enantiomeric species but also possess advantageous features such as ease of production, low-cost, high sensitivity and adaptation, have drawn substantial interests of researchers [15]. The key requirement of optical approaches to assess enantiomeric contents is chiral sensors that capable of differentially interact with antipodes of a chiral analyte in a manner that results in different optical responses. With the recent major developments in molecular sensing and supramolecular chemistry, varieties of enantioselective optical sensors such as small molecule, metal complex and polymer-based enantioselective sensors have been fabricated over the past decades [15]. With the recent advances in nanotechnology, nanomaterials have become an alternative and ideal constituents for the construction of optical sensing systems [16]. Noble metal nanomaterials, particularly silver nanoparticles (AgNPs) and gold nanoparticles (AuNPs), are extensively used as substances to build up an optical sensor owing to their size and shape depending optical properties [17]. In recent years, the researchers have devoted considerable eff ort to the production and application of AgNPs and AuNPs for the field of
4
optical chiral discrimination due to their inherent chirality leading to enantioselective interaction towards antipodes of chiral species [18–25]. In the last decade, nanomaterial-incorporated paper-based sensors have attracted marvellous research and commercial attention due to their applicability in design of various disposable instruments for consumers [26–31]. Additionally, these kind of optical sensing platforms can easily adapt the visual detection to simple optical/electrical readers, thus no complicated devices are required [25]. As an optical sensing platform, nanopaper defined as a sheet made of cellulose nanofibers (below 50 nm) has recently been a new focus in paperbased sensors due to its unique advantages and numerous desirable properties such as high optical transparency and mechanical properties, flexibility, environmental sustainability and biodegradability [32,33]. The transparent nature of the nanopaper also makes it a very suitable substrate for incorporation or immobilization of optical sensing agents to be utilized in optical sensors [34]. Considering the
aforementioned
content,
the
silver
nanoparticles-embedded
transparent nanopaper was proposed as an optical chiral sensing platform in this study. Initially, the chiral recognition capability of silver nanoparticles was examined in solution phase. Then, the advantages of nanopaper and AgNPs were combined for the development of a disposable nanopaper-based sensor. To this aim, nanopaper was obtained by an environment-friendly approach using bacterial cellulose made of nanofibers. Silver nanoparticles were in-situ decorated within nanopaper as a result of direct reduction of silver ions by hydroxyl groups of nanofibers, without any external derivatization and reducing agents. After the operational principle of this nanopaper-based assay and its proficiency to be utilized in such an significant area were assessed, single use, robust and cost-effective waxprinted two dimensional (2D) cuvettes were developed for practical applications. By operating wax-printed cuvettes, not only to minimize utilization of the sample without any
5
exposure but also decrease the sample volume (microliters) was succeeded and made it more convenient for small volume sample analysis in an ordinary spectrophotometer. In this study, the colorimetric discrimination of enantiomers was examined and a satisfactory response was obtained for cysteine enantiomers. In the literature, several papers were reported on the cysteine (no defined D-/L- cysteine, or L-cysteine) detection using the method of aggregation of Ag or Au nanoparticles [35–41]. Differently from these studies, herein, discriminative sensing of D-/L-cysteine enantiomers was proposed by the use of AgNPs-embedded transparent nanopaper as a novel sensing strategy without the need of advanced instruments.
2. Material and methods 2.1. Reagent and apparatus Silver nitrate (AgNO3), trisodium citrate (Na3C6H5O7), NaBH4, D- and L-enantiomers of the tested chiral species and glucose, (NH4)2SO4, KH2PO4, MgSO4.7H2O, NaOH, H2O2 were purchased from Sigma-Aldrich. A JEOL JEM 2100 UHR-TEM was used for Transmission Electron Microscopy (TEM) images. Scanning electron microscopy (SEM) was performed by a ZEISS EVO LS 10 SEM. A Shimadzu UV-1800 double beam spectrophotometer was employed for UV-Vis absorption measurements. A ColorQube 8580 wax printer was used to fabricate the disposable cuvettes. The photographs were taken with a Canon EOS 700D digital camera. A Milli-Q system (Millipore) was used to obtain ultrapure water.
2.2. Fabrication of Bacterial Cellulose Nanopaper
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Bacterial cellulose nanopaper was produced by an environment-friendly method [32]. Briefly, Acetobacter xylinum bacteria culture was incubated in 1 liter of water containing glucose (50 g), yeast extract (5 g), (NH4)2SO4 (5 g), KH2PO4 (4 g) and MgSO4.7H2O (0.1 g) at 28 ˚C for two weeks. Then, the resultant (3-4 mm-thick unpurified wet and gel-like mat made of cellulose nanofibers) was first treated with 5 wt% NaOH for 24 h at ambient conditions and then with 1wt% NaOH/0.2 wt% H2O2 for 1 h at 80 ᵒC in order to purify from bacteria and the other impurities. As the second purification step, the produced wet mat made of bacterial cellulose nanofibers was washed with ultrapure water to remove the chemicals and to obtain a transparent nanopaper (Fig. S1). 2.3. Preparation of the silver nanoparticles (AgNPs) All materials used in the preparation process were cleaned with aqua regia (HNO3/HCl= 1:3, v/v) and washed with ultrapure water. Silver nanoparticles were obtained using AgNO3, and NaBH4 and trisodium citrate (TSC). The synthesis process was operated in the dark conditions to prevent photodecomposition of AgNO3. Briefly, a 62,5 μL of 0,1M AgNO3 was added into 25 mL of pure water. A 62,5 μL of 0,1M TSC was added to the resultant solution and mixed for 30 min under constant stirring at ambient temperature. Then, 1.5 mL of 0.005 M NaBH4 was incubated to the solution and the color immediately changed to light yellow and finally to amber expressing the formation of silver nanoparticles [42]. The resultant was vigorously stirred for 2h. The all procedure for the synthesis of AgNPs was shown in Fig. S2a. 2.4. Fabrication of in-Situ Generated AgNPs-embedded Nanopaper The procedure for fabrication of AgNPs-embedded nanopaper was performed by following previous literature [34]. Briefly: A 62,5 μL of 0,1M TSC was added to 20 mL of ultra pure water (pH 8.0) in a glass bottle. A piece (1 cm × 2 cm) of as-prepared wet bacterial cellulose nanopaper was added. Then, the resultant was heated (50 °C) and stirred for 15 min. 7
Then, a 5 mL of AgNO3 (0.01 M) was syringed dropwise to mixture under the ambient conditions with vigorous stirring. Afterwards, pH value of the resultant was arranged to 12.0 and it was continuously stirred at 50 °C for 2 h. The preparation of AgNPs-embedded nanopaper was preliminarily assessed with gradually change in colour from transparent to yellow-amber expressing the formation of silver nanoparticles (Fig. S2b). Afterwards, the resultant was cooled to ambient temperature. AgNPs-embedded nanopaper was separated from the solution and washed with ultrapure water to remove the precursors from the fabricated nanopaper surface. The nanopaper was punched into the circular pieces for practical use (Fig. S2c). Finally, they were stored in a closed vessel in refrigerator. It should be remarked that AgNPs-embedded nanopapers remain undestroyed for more than 12 weeks. The fabrication of AgNPs-embedded nanopaper was confirmed not only by UV-visible absorption spectra and also by SEM, EDX and elemental mapping analysis. Fig. S3 shows the EDX and elemental mapping analysis results. It is clearly seen that AgNPs-embedded nanopaper shows homogenously distributed AgNPs in elemental mapping at the selected area of the corresponding SEM image with Ag peak beside C and O peaks in EDX data indicating the presence of AgNPs over the blank nanopaper. 3. Results and discussion 3.1. The Enantioselective Response Towards Cysteine Enantiomers Noble metal nanoparticles have been reported to offer potential for chiral sensing applications due to their plasmonic properties. As initial demonstration of the potential of AgNPs for optical enantioselective sensing use, the classical colorimetric experiments have been performed in solution depending on a colour change which is a simple but efficient way fairly investigated in the literature. This operational principle has been successfully performed in various works. To illustrate, Zhang and Ye have proposed a surface modified-Ag nanoparticles (AgNPs) for visual detection of D- and L-cysteine [24]. Liu et al. have reported 8
a simple and convenient procedure for chiral recognition of aromatic amino acid with β-CD coated AgNPs by taking advantage of favorable features of β-CD (surface coating agent) and the distance dependent optical property of AgNPs (colorimetric probe) [19]. Apart from the surface-functionalized forms of AgNPs, Liu et al. have demonstrated that the citrate-capped AgNPs can be utilized as optical probe in chiral recognition of tryptophan enantiomers by using advantage of the inherently chiral AgNPs [18]. Within this respect, the overall optical enantioselective performance of unfunctionalized AgNPs was explored. To this aim, the solution including excess quantity of the used chiral species was mixed with as-prepared AgNPs solution. The change in colour was examined by naked-eye. As shown in Fig. 1a, a distinct optical change from yellow to purple with the addition of D-cysteine. Interestingly, no distinct difference was obtained in colour for L-cysteine, which indicates that AgNPs displayed enantioselective interaction towards one enantiomer of cysteine. A centrifugation process was applied to the mixed solutions of enantiomers and AgNPs. It was observed that a large quantity of precipitate occurred for the existence of D-cysteine, which was ascribed to the aggregation of AgNPs (see Fig. 1a'). Besides these observations, both enantiomers of the rest of the used chiral species have displayed either a significant or no optical response. This result indicates that both antipodes act in the same manner leading to no selective enantiorecognition process (Fig. S4). It can be ascribed to (i) both antipodes conveniently interact with AgNPs, or not; (ii) the dimentional position of the interaction and steric hindrance effecting the internanoparticle distance of AgNPs. Herein, the elucidation of enantioselective recognition mechanism for the occurring process is the major point and it is also rational to note that this AgNPs-based optical sensor is efficient for enantioselective detection of cysteine enantiomers. Within this respect, in the previously proposed chiral sensing assays involving AuNPs and AgNPs [18–25], the major element has been attributed to the enantioselective groups attached to the nanoparticles surface. On the other hand, metal
9
surfaces [43], AuNPs and AgNPs are inherently chiral species [17,44,45]. Considering the aforementioned reasons above, associated with our study, Zhang et al have developed a chiral sensor for cysteine enantiomers in aqueous sample by AgNPs modified with uridine 5´triphosphate (UTP) as enantioselective unit. In our method, the chiral recognition can be explained by inherent chirality arisen through the kink sites in the framework of metal nanoparticle (inherently chiral AgNPs) [43]. While citrate molecule does not exhibit chirality, the backbone of the citrate-capped AgNPs can endow a convenient enantioselective interaction and/or chemistry [18,24]. Moreover, convenient dimensional interaction of cysteine-adsorbed AgNPs leads to aggregation upon interaction resulting to the purplecoloured solution. Finally, the possible mechanism can be elucidated by the interaction of Dcysteine molecules which may interact with AgNPs by means of thiol (-SH) group or amino group (−NH2) with larger adsorption energy than L-cysteine molecules. After that, the interaction of at least two D-cysteine molecules can occur between their spatially neighboring carboxylic groups (−COOH) at the chiral centers leading to the most suitable dimensional interaction via hydrogen bonds. This defeats the electrostatic and steric hindrance, and finally reduce the distance between AgNPs to accumulate. As a consequence, the aforementioned results indicate that AgNPs exhibit good enantioselectivity towards D-cysteine.
Here Figure 1
Aiming at investigating the effect of pH values on enantioselective determination of cysteine enantiomers, the behavior of AgNPs was examined for pH range of 3.0-8.0. As indicated in the previous literature [21,46], AgNPs were not stable and tend to aggregate at extreme acidic and alkaline pH. In our system, AgNPs aggregated even without cysteine at
10
pH < 5.0 and pH > 8.0, and the optimum pH value was obtained to be pH 6.0 for stable AgNPs and enantiorecognition of cysteine enantiomers. As the second demonstration of enantioselective recognition of cysteine enantiomers, AgNPs solution was utilized as a colorimetric probe and the changes were followed by UVVis spectrophotometry technique in the absence and presence of cysteine enantiomers. Fig. 1b shows UV-Vis spectra of blank AgNPs solution, D-cysteine/AgNPs and L-cysteine/AgNPs solutions. A maximum absorption peak was obtained at 408 nm arising from the surface plasmon absorption of AgNPs [47]. It was red-shifted by addition of D-cysteine while only a small decrease was monitored for L-cysteine, which remarks that D-cysteine could enantioselectively bounds to AgNPs, which leads to aggregation of AgNPs, but L-cysteine displays no significant effect, as demonstrated by the colorimetric experimental results. Aiming at examining the analytical applicability of AgNPs as colorimetric probes for the detection of chiral species, the effect of cysteine enantiomers concentration on the absorption profile was assessed. Fig. 1c and 1d shows the absorption spectra of AgNPs solution displaying optical response for D- and L-cysteine. The peak at 408 nm decreased accompanied with a gradually increment of peak at 600 nm for increasing concentrations. Different from this observation for D-cysteine, no significant change on the AgNPs was observed as for increasing concentration of L-cysteine.
3.2. Application of AgNPs-embedded nanopaper for colorimetric chiral sensing Considering the results of colorimetric and spectrophotometric experiments, the applicability of a single use paper-based chiral assay using AgNPs-embedded nanopaper for practical applications have been foreseen. To assess the potential of functionalized plasmonic nanopaper for optical sensing use, AgNPs-embedded nanopaper was utilized as a colorimetric enantioselective sensor for cysteine enantiomers. Fig. 2a illustrates the change in color of
11
AgNPs-embedded nanopaper from yellow to purple-brown upon addition of different amount of D-cysteine, whereas no significant change was observed in color for addition of L-cysteine. After 100 µM D-cysteine, no change was obtained in color indicating the saturation of the sensing response.
Here Figure 2
Scheme 1 shows the overall experimental procedure and the operational concept of the developed nanopaper-based sensor. The hydrophilic character of nanopaper allows the adsorption of solution and the inherent chirality of AgNPs allows the enantioselective interaction with the enantiomers. Once the sampling of D-cysteine solution (50 µL) onto AgNPs-embedded nanopaper, the color changes through aggregation-induced by interaction of D-cysteine molecules with each other.
Here Scheme 1
For quantitative evaluation, the colour density is converted to the colour intensity (CI) percentage scale (defined as the analytical signal) by using ImageJ software. Fig. 2b displays the linear increase in colour intensity, CI, after adding different concentrations of D-cysteine (as C) between 5 µM and 100 µM. As experimental evidences, the performance of assay linearly changes (R2 = 0.993) which can be estimated by Equation 1: CI = 0.48 (±0.04) C (D-cysteine) + 12.01 (±0.71) (n=3)
(1)
12
where n is the number of performed sample. Limit of detection (LOD) value was calculated to be 4.88 µM for D-cysteine. LOD could not be estimated for L-cysteine because no distinct change was obtained at CI in the presence of different concentration of L-cysteine. As observed in the colorimetric experiments, D-cysteine can be selectively adsorbed onto AgNPs and it can act as a linker on AgNPs, thereby promoting their aggregation. This phenomenon can be seen in the TEM images in Fig. 3a-a', 3b-b' and 3c-c'. As shown in Fig. 3a-a', citrate-capped AgNPs (18 ±2 nm, spherical-shaped) were homogenously dispersed in the solution without D-cysteine or L-cysteine. Correspondingly, L-cysteine addition (Fig. 3bb') could not induced a significant aggregation of AgNPs, as evidenced in the aforementioned solution phase experiments. However, large aggregates appeared after addition of D-cysteine that could be expressed by bridge-like role of D-cysteine enantiomers (on the surfaces of AgNPs) drawing the distance between AgNPs by interaction aforementioned above and therefore giving rise to remarkable aggregation as shown in Fig. 3c-c'. Differently from the reported papers which demonstrated that several metal ions would be utilized as cross-linking agents to induce the agglomeration of nanoparticles [23,48], herein it was demonstrated that the aggregation could be directly achieved in the absence of any cross-linkers which would render the process more complicated by increasing uncertainty to understand the entire process.
Here Figure 3 The same phenomenon was observed not only in the solution phase but also on nanopaper. Fig. 3d-d', 3e-e' and 3f-f' show SEM micrographs, which respectively correspond to the prepared nanopaper and AgNPs-embedded nanopaper after adding D-cysteine and Lcysteine at low and high magnifications. The diameter of nanofibers were measured on SEM micrographs and was observed as 40 ± 10 nm. Fig. 3e-f show how D-cysteine and L-cysteine
13
affect AgNPs in nanopaper at the nanoscale. On the basis of our findings, it can be concluded that D-cysteine facilitate the aggregation of AgNPs in nanopaper, as well.
3.3. The performance of AgNPs-Embedded Nanopaper in Determination of Enantiomeric Percentage One of the essential issues in chiral detection is to examine enantiomeric percentage by the proposed platform, because enantiomers of a chiral molecule generally coexist in an environment. For this purpose, utilizing the developed assay, AgNPs-embedded nanopaper could be directly employed for determination of the percentage of cysteine in a racemic solution. As observed in Fig. S5a-b, the aggregation of AgNPs was induced by D-cysteine, and a change in color intensity was observed for different enantiomeric percentages of Dcysteine between 5% and 100% for the optimized sensor. Another interesting point to be mentioned was whether the selective sensing performance of AgNPs-embedded nanopaper would be investigated in a common spectrophotometer by using a disposable microcuvette because the AgNPs-embedded nanopaper exhibited high transparency behavior before and after sampling for enantiomeric percentage of D-cysteine in a mixed solution (see Fig. S5c). In our previous paper, we have developed a sensing system based on AuNPs-employed assay consisting of a conjugation membrane capturing the target and the detection membrane signaling the existence of the captured target [25]. The proposed paper-based device was a convenient platform for chiral sensing applications, while it possessed some drawbacks such as non-transparency of the used cellulose acetate membranes and the requirement of high volume of sample ( at least 2 mL). In this study, as for decrease the sample volume for the use in small volume of samples, the microcuvette modulation for spectroscopic measurements at solid platform was performed by using transparent AgNPs-embedded nanopaper with a very small volume of sample (50 µL) for the same percentage range of cysteine enantiomers, see
14
Fig. 4a. Our initial design criteria for the microcuvettes was the size and optically-inactive disposable platform, and thus control the overall absorption upon interaction of AgNPs and D-cysteine. Here Figure 4
As aforementioned, wax printed PET film was used as holders which were opticallyinactive and could be easily attached onto holder of spectrophotometer with a stabilized position. This preparation and easy modulation procedure of sensing assay was shown step by step in Fig. S6. In our sensing system, because of the requirement of entirely wetting of the nanopaper, an adjusted volume of sample (50 μL) was required to assure efficiently capturing of analyte which led to absorption change in confined aqueous solution by nanopaper. After this process, the treated nanopaper was finally placed onto the microcuvette to perform spectrophotometric measurements using an ordinary spectrophotometer. Fig. 4b reveals the UV-Vis spectra of microcuvettes consisting of AgNPs-embedded nanopapers for the enantiomeric percentage of D-cysteine within the operational range between 5% and 100%. As clearly seen, similar to the solution phase experimental results, an absorption peak was obtained at 408 nm arising from the surface plasmon absorption of AgNPs. The absorption peak was red-shifted (to 600 nm) at different enantiomeric percentage of D-cysteine which demonstrated that the developed microcuvette platform was a simple and rapid approach for sensitive and selective optical sensing of enantiomeric percentage of D-/L-cysteine. 4. Conclusion A simple, versatile and disposable platform has been developed for enantiosensing applications of chiral molecules by exploiting the impressive properties of nanopaper and AgNPs. Qualitative analyses have been performed by simply treating AgNPs with cysteine enantiomers and observing (with the naked eye) the generation of a change in colour of 15
AgNPs. By taking advantage of the inherently chiral yellow-coloured AgNPs that displays a colour change to purple-brown in the existence of D-cysteine, enantioselective recognition of cysteine enantiomers has been successfully achieved in solution. Also, it has been shown that this
enantioselective
platform
can
be
easily
adapted
to
useful
devices
(e.g.,
spectrophotometer) for individual assays by using simple punch tools and wax printing for holder preparation. By using this feature, quantitative measurements have been taken to determine enantiomeric percentage of cysteine enantiomers by using an ordinary spectrophotometer. Overall, this class of platforms can opens up innovative capabilities to design not only novel enantiosensing samples analysis strategies but also diagnostics, environmental monitoring and food safety systems, which can be readily analyzed by a simple software controlled via a camera supported devices, such as mobile phone or tablet computer. Appendix A. Supplementary material Supplementary data associated with this article can be found in the online version.
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Scheme Legend Scheme 1. Schematic representation of the experimental procedure and operational concept of nanopaper-based platform (not to scale) as enantioselective optical assay based on aggregation-induced by sampling of D-cysteine. 20
Figure Legends Fig. 1. The colour change of AgNPs solution with and without 100 μM of D- or L-cysteine (a). The centrifuged AgNPs solutions including of D- or L-cysteine (a'). UV-Vis spectra of as-synthesized AgNPs, with and without 100 μM of D- or L-cysteine (b). UV–Vis titration spectra for D-cysteine (c) and L-cysteine (d) between 0 and 100 µM. Fig. 2. The nanopapers before and after sampling D- and L-cysteine enantiomers between 5 and 100 µM (a). The change of colour intensity upon the presence of D- and L-cysteine between 5 and 100 µM for the optimized assay (b). Fig. 3. TEM images of blank AgNPs (a-a') (at different resolutions), with L-cysteine (b-b'), and D-cysteine (c-c') enantiomers. SEM images of nanopaper at different magnifications at 1 µm (d) and at 200 nm (d'), respectively. SEM images of AgNPs embedded-nanopaper in the presence of L-cysteine (e-e'), and D-cysteine (f-f') enantiomers at low (1 µm) and high (200 nm) magnification, respectively. Fig. 4. The microcuvette modulation for spectrophotometric measurements at solid platform for the same enantiomeric percentage range of D-cysteine (a). The recorded UV-Vis spectra for the corresponding microcuvettes (b).
Scheme 1
21
Figure 1
22
Figure 2
23
Figure 3
24
Figure 4
25
Highlights
AgNPs-embedded nanopaper is developed for enantioselective sensing of chiral molecules.
Colorimetric discriminative sensing of cysteine enantiomers is achieved.
The enantiomeric percentage can be determined by the proposed sensor.
26