Adsorption of 1,4-phenylene diisothiocyanate onto the graphene oxide sheets functionalized with polydiphenylamine in doped state

Adsorption processes of 1,4-phenylene diisothiocyanate (PDITC) on two new platforms of the type graphene oxide (GO) sheets and GO layers functionalization with polydiphenylamine (PDPA) are studied by Raman scattering and photoluminescence (PL). An interaction in solid state phase of the two constituents, i.e. PDITC and GO sheets, and a deposition of PDITC onto the PDPA functionalized GO layers, respectively, by the drop casting method, were performed. In the first case, it is shown that interaction in solid state phase of GO with PDITC leads to an intercalation of the organic compound between GO sheets simultaneously with the appearance of the o-thiocarbamate groups, that induces: (i) an enhancement of the PDITC Raman lines situated in the 400–800 and 1000–1300 cm−1 spectral ranges, (ii) a change in the ratio between the relative intensities of the two Raman lines peaked at 1585 and 1602 cm−1 accompanied by an up-shift in the case of the second line and (iii) a down-shift of the PDTIC PL band from 502 to 491 nm. Using cyclic voltammetry, an electrochemical functionalization of the GO layers with PDPA doped with H3PMo12O40 heteropolyanions takes place, as demonstrated by Raman scattering and FTIR spectroscopy. The presence of the amine groups in the molecular structure of the doped PDPA functionalized GO layers induces a chemical adsorption of PDITC on this platform, when the thiourea groups appear simultaneously with o-thiocarbamate groups. A chemical mechanism is proposed to take place at the interface of the GO sheets and the doped PDPA functionalized GO layers, respectively, with PDITC.

Organic compound 1,4-phenylene diisothiocyanate (PDITC) is one among from the often used coupling agents (cross-linkers) for biological applications performed in the presence of various surfaces modified with amines of the type ethylenediamine 1 , cysteamine 2,3 , 3-amino-propyl(triethoxysilane) 4 and so on. Applications of PDITC in the field of electrochemical immunosensors, for the detection of markers such as the epidermal growth factor receptor 1 and Murine double minute 2 2 in the brain tissue or human plasma, have involved a chemical adsorption of PDITC onto the gold nanoparticles/electrodes surface modified with cysteamine. The high cost of the Au nanoparticles/electrodes is a major drawback for the manufacturing of such platforms at large-scale. In order to overcome this inconvenient, in the present work, the GO sheets and the GO layers functionalized with polydiphenylamine (PDPA) are characterized. In this work, the attention will be focalised on the understanding of the PDITC adsorption mechanism during the interaction in the solid-state phase of PDITC with the GO sheets and the deposition by drop casting method of PDITC onto the GO layers functionalized with PDPA. A study concerning the adsorption/interaction of the PDITC on/with GO sheets or their composites has not been reported until now. In this work, the main aim consists in the understanding of the adsorption mechanism of PDITC onto the GO sheets functionalized with PDPA in doped state. Thiocyanate adsorption was studied using Ag nanoparticles 5 , Au films modified with cysteamine 6 and the Au-Pd core-shell nanoparticles 7 . Optical methods often used in evaluating PDITC adsorption on the metallic nanostructures include surface enhanced Raman scattering, IR spectroscopy and atomic force microscopy 5,6 .
In this work, the Raman scattering will also be used for assessing the PDITC adsorption on the GO sheets and the PDPA functionalized GO layers. The photosensitivity of PDITC will be highlighted by PL in all subsequent

Results and Discussions
Optical properties of the GO sheets interacting with PDTIC. Figure 1a shows the Raman spectrum of GO, which is characterized by two bands peaked at 1346 and 1592 cm −1 , assigned to the hexagonal rings breathing vibrational mode and the E 2g phonon mode at the Brillouin zone centre 8 . Figure 1b shows the Raman spectrum of PDITC, characterized by the three high intensity lines situated in the 1000-1650 cm −1 spectral range and other five lines of low intensity localized in the 300-700 and 2000-2200 cm −1 spectral ranges. The PDITC Raman lines, peaked at 368, 434, 632-695, 1157, 1257, 1583, 1603 and 2080 cm −1 , are assigned to the following vibrational modes: deformation of a p-substituted benzene ring 9 , bending deformation of the NCS bond 10 , asymmetric C-S stretching 11 , C-S bending 11 , C-H in benzene ring + C-C stretching + C-N stretching 9 , C=C + C-C stretching in benzene ring 9,12 , C-C stretching + C-H bending in benzene ring 9 and C=N stretching 5 , respectively. The following differences are observed in the Raman spectra of the platelets of PDITC with 0, 1 and 2 wt.% GO (Fig. 1b-d): (i) an enhancement in the relative intensities of the PDITC Raman lines situated in the 400-800 and 1000-1300 cm −1 spectral ranges, when the concentration of GO in the PDITC platelets weight is equal to 1 and 2 wt.% (Fig. 1c,d); (ii) a decrease in the ratio between the relative intensities of the two Raman lines peaked at 1583 and 1603-1616 cm −1 (I 1583 /I 1603-1616 ) from 5.29 (Fig. 1b) to 2.82-2.97 (Fig. 1c,d) with increasing the GO concentration in the PDITC platelets weight; this change is accompanied of an up-shift of the Raman line assigned to the vibrational mode of C-C stretching +C-H bending in benzene ring, from 1603 to 1616 cm −1 , when the GO concentration in the PDITC platelets weight increases from 0 to 2 wt.%; iii) the ratio between the relative intensities of the Raman lines peaked at 1583, 1255-1257 and 1157 cm −1 (I 1583 /I 1255-1257 and I 1583 /I 1157 ) decrease from 0.71 and 1.1 (Fig. 1b) to 0.39 and 0.41 (Fig. 1c) or 0.34 and 0.27 (Fig. 1d), as the GO concentration increases from 0 to 1 and 2 wt.%; iv) a significant decrease in the ratio between the relative intensities of the Raman lines peaked at 1583 and 366-368 cm −1 (I 1583 /I 366-368 ) from 8.82 (Fig. 1b) to 0.63 (Fig. 1c) and 0.56 (Fig. 1d), when the GO concentration in the PDITC platelets weight increases from 0 to 2 wt.%; and v) a gradual up-shift of the D band of GO from 1346 cm −1 (Fig. 1a) to 1348 cm −1 (Fig. 1c) and 1352 cm −1 (Fig. 1d) occurs in the presence of PDITC.
All these changes indicate the presence of a steric hindrance effect that can be explained only if we accept that an interaction between PDITC and GO takes place according to Fig. 1S. Figure 1S indicates the formation of two compounds, the former corresponding to the GO sheets intercalated with PDITC (GO-PDITC-GO) which shows o-thiocarbamate groups and a second one which illustrates the GO sheets modified with PDITC (GO-PDITC) having a molecular structure which contains both of the o-thiocarbamate functional groups and the isothiocyanate terminated surface. The formation of the GO-PDITC-GO compound explains the enhancement of the PDITC Raman lines situated in the 400-800 and 1000-1300 cm −1 spectral ranges.
Additional information concerning the interaction of the GO sheets with PDITC is obtained by IR spectroscopy. Figure 2 shows the IR spectra of PDITC and the GO sheets before and after their interaction in solid state phase. According to Fig. 2, the main IR bands of the GO sheets are peaked at 1041, 1230, 1373, 1614, 1724 and 3604 cm −1 , these being assigned to the following vibrational modes: C=C, OH groups from the GO sheet edges, alkoxy groups, H 2 O adsorption onto the GO sheets' surface, C=O groups belonging COOH and OH stretching 8 . The most intense IR bands of PDITC are peaked at 827 and 1489 cm −1 , these being assigned to the vibrational modes of bending of C-H out of plane in benzene p-substituted and -C=N-benzene ring, respectively 9,13 . The interaction of PDITC with the GO sheets leads to the following changes in the IR spectra of the two constituents: (i) a decrease in the ratio between the absorbance of the two IR bands of PDITC peaked at 1489 and 827 cm −1 from 0.71 (green curve in Fig. 2) to 0.43 (red curve in Fig. 2) and (ii) an increase of the ratio between the absorbance of the IR bands belonging to the GO sheets peaked at 1724 and 3604 cm −1 , from 5.4 (green curve in Fig. 2) to 9.84 (blue curve in Fig. 2) and 23 (red curve in Fig. 2) simultaneously with a more pronounced decrease in the absorbance of the IR band at 3604 cm −1 . The decrease of the absorbance of the IR band at 1489 cm −1 indicates a smaller weight of the -C=N-benzene ring vibrational mode after the interaction of the GO sheets with PDITC, this being in good agreement with the processes shown in Fig. 1S. In addition to above changes, a down-shift of the PDTIC PL band from 549 nm to 511 nm is observed, as seen in Fig. 3. An interesting fact shown in Fig. 3 is the change of the PL spectra of PDITC and that of the GO sheets intercalated with PDITC, when the excitation wavelength is equal with 375 nm. When increasing the irradiation time to 110 min., a gradual increase is observed in the relative intensity of the PL bands of PDITC and the GO sheets intercalated with PDITC, from 423.532 and 405.404 counts/sec to 840.388 and 947.136 counts/sec, respectively. This change is accompanied by a down-shift of the PL band maximum of PDITC and the GO sheets intercalated with PDITC from 549 and 511 nm at 502 and 492 nm, respectively. In our opinion, these variations indicate a photochemical process, induced by water vapours form air, as shown in Fig. 2S for PDITC. A similar reaction can be invoked for the GO-PDTIC compound, formed through the interaction of PDITC with GO according to Fig. 1S. PL spectra remained unchanged for four hours, when recorded in vacuum (at the pressure of 5.4 × 10 −6 mbar). The difference in the value of the PL band maximum shift is due to the small number of the isothiocyanate groups in the compounds resulted from the interaction of the GO sheets with PDITC. Summarizing all these results, we can conclude that: i) interaction in solid state phase of GO with PDITC leads to an intercalation of the organic compound between GO sheets, leading to the generation of new o-thiocarbamate functional groups, and ii) the manipulation of PDITC and its derivates must to be carried out in the absence of UV light in order to avoid occurrence of photochemical reactions.
Optical properties of GO sheets functionalized with PDPA before and after the PDITC deposition. Figure  www.nature.com/scientificreports www.nature.com/scientificreports/ increasing in the absorbance of the IR band peaked at 1591 cm −1 is noted as a consequence of a greater weight of PDPA compared to the GO layer surface. As shown in our previous work, the ratio between the absorbance of the IR bands of standalone PDPA peaked at 698, 752 and 1028 cm −1 (A 698 /A 1028 and A 698 / 1028 ) is equal to 7.1 and 6.9, respectively 21 . These IR bands were assigned to the vibrational modes of inter-ring deformation, benzene ring deformation and A 1 benzene 9,21 . According to Fig. 5, in the case of the SPCE modified with a GO layer as the DPA concentration in the synthesis solution increases from 10 −3 M to 5 10 −3 /10 −2 M, one observes that: (i) the vibrational modes of inter-ring deformation, benzene ring deformation and A 1 benzene are down-shifted at 692, 746 and 1020 cm −1 ; (ii) the values of the A 692 /A 1020 and A 746 / 1020 ratios are changed from 0.39 and 0.43 to 0.68/0.95 and 0.86/1.15, respectively and (iii) the ratio between the absorbances of the IR bands peaked at 945 and 1020 cm −1 is changed from 0.39 to 1.23 and 1.67. The higher absorbance of the IR bands assigned to the deformation vibrational modes of the benzene and quinoid rings as well as inter-rings highlights the significant steric hindrance effects induced by the covalent bonding of PDPA doped with H 3 PMo 12 O 40 heteropolyanions onto the GO layers. Figure 6 shows the Raman spectra of the GO layers covalently functionalized with PDPA in doped state, after the deposition of PDITC from a solution of PDITC in C 2 H 5 OH having the concentration 0.1 mg/ml, by the evaporation of 1 ml (a) and 2 ml (b) solvent. The GO layers electrochemical functionalized with PDPA in doped state were prepared using a semi-aqueous solution of 10  www.nature.com/scientificreports www.nature.com/scientificreports/ this platform, when the thiourea groups appear simultaneously with those of the type o-thiocarbamate. Figure 5S shows the reaction of PDITC with the doped PDPA functionalized GO layers. A similar activation of amino groups of the working electrode by interaction with PDITC was reported in the case of: (i) cysteamine modified Au electrode 2 , (ii) 11-Mercaptoundecanoic acid modified Au electrode 22 ; and (iii) β-cyclodextrin -reduced graphene oxide-tetraetylene 23 . An optical image of the samples of the doped PDPA functionalized GO layers after the PDITC adsorption is shown in Figure 6c 1 and 6c 2 .These images highlight the formation of a one-dimensional macrostructures as a consequence of the generation of the thiourea groups by the PDITC adsorption onto the doped PDPA functionalized GO layers. The presence of isothiocyanate groups onto the surface of the GO sheets and that of the GO layers functionalized by PDPA, which were intercalated/modified with PDITC, open new perspectives on the applications of these platforms in the immunosensors field for the detection of the cancer markers, for example.
We note that samples resulted after the deposition of PDITC onto the GO layers functionalized by PDPA must be handled in the absence of UV light. In order to support this sentence, Fig. 8 shows the evolution of PL spectra of the sample obtained after the deposition of PDITC from a solution of PDITC in C 2 H 5 OH having the concentration of 0.1 mg/ml, by the evaporation of 2 ml solvent onto the GO sheets functionalized with PDPA  and 1 M HCl in DMF: H 2 O, when SPCE modified with GO was used as working electrode, leads to the GO layers covalently functionalized with PDPA in doped state; iii) the adsorption of PDITC onto the GO layers covalently functionalized with PDPA in doped state involves the appearance of the thiourea groups simultaneously with those of the type o-thiocarbamate. iv) a photochemical process was reported in the case of the PDITC adsorption onto the GO sheets surface as well as the GO layers covalently functionalized with PDPA in doped state. These results indicate the necessity of the manipulation of these platforms in the absence of the UV light.  8 .

Methods
In the present work, H 3 PMo 12 O 40 acts as an initiator for the growth of macromolecular chain of PDPA onto the GO sheets and a doping agent of PDPA. The GO layers functionalized with PDPA doped with H 3 PMo 12 O 40 heteropolyanions were prepared by electrochemical polymerization of DPA onto the screen-printed carbon electrodes (SPCE) modified with GO purchased from DropSens. The cyclic voltammetry studies were carried out by the immersion of the SPCE into a solution consisting from DPA (10 −3 , 5 10 −3 or 10 −2 M), 10 −3 M H 3 PMo 12 O 40 and 1 M HCl in semi-aqueous solution of DMF:H 2 O having the volumetric ratio of 1:1. The potential range was between +100 and +960 mV vs. Ag electrode and a sweep rate equal with 50 mV s −1 was used. The reported cyclic voltammograms using SPCE were recorded with a potentiostat/galvanostat, Voltalab 80 model, purchased from Radiometer Analytical.
The interaction in solid state phase of PDITC with the GO sheets was carried out in the absence of light by the mechanico-chemical reaction of the two constituents that were compressed non-hydrostatically at the pressure of 0.58 GPa, time of 5 minutes, resulting in platelets of PDITC with 1 and 2 wt.% GO.
The PDITC deposition on the PDPA functionalized GO layers was performed by the drop casting method using a solution of PDPA in C 2 H 5 OH having the concentration of 1 mg/ml.
Raman spectra of PDTIC, GO sheets, PDPA intercalated GO sheets and PDPA functionalized GO layers before and after the interaction with PDTIC were recorded under the excitation wavelength of 514 nm using a spectrophotometer Raman, T64000 model, from Horiba Jobin Yvon, endowed with an Ar laser.
The PL spectra of PDTIC, PDPA intercalated GO sheets and PDPA functionalized GO layers before and after the interaction with PDTIC were recorded at room temperature, under the excitation wavelength of 375 nm and 275 nm, respectively, with a spectrophotometer Fluorolog-3.2.2.1, from Horiba Jobin Yvon.
The IR spectra of GO sheets before and after the interaction with PDITC were recorded with a FTIR spectrophotometer, Vertex 80 model, from Bruker.