Immobilization on Cellulose Paper of a Chemosensor for CdSe-Cys QDs †

: The Schiff base ligand H 2 SB derived from the condensation of N -(2-aminobenzyl)-5- (dimethylamino)naphthalene-1-sulfonamide with 4-formyl-3-hydroxybenzoic acid has been investigated as a chemosensor for the detection of CdSe-Cys QDs in water samples. We immobilized H 2 SB onto cellulose paper by forming an amide bond, which results from the condensation of a carboxylic acid and an amine. Three dominant signals located around 270, 330, and 420 nm in the diffuse reﬂectance spectrum of the H 2 SB-modiﬁed paper demonstrated its immobilization. A linear decrease can be observed in the absorbance of the 270 nm band with the increase of the CdSe-Cys QDs concentration from 100 ppb to 2 ppm. The LOD and LOQ show values of 245 and 815 ppb, respectively. An interaction via metal–ligand coordination between CdSe-Cys QDs and H 2 SB has been demonstrated with 1 H NMR, ATR-FTIR, and UV-Vis spectroscopy.


Introduction
Cadmium-based QDs are one of the most widely used semiconducting QDs, as they have useful properties for biochemical sensors, biomedical imaging, photovoltaic applications, light-emitting diodes (LEDs), thin-film transistors, lasers, and solar cells [1,2]. A negative consequence of this widespread use is the release into the environment of such noxious material. To prevent their uncontrolled discharge, it is essential to develop simple methods to detect them in waste or even in the environment. The implementation of paper sensors for colour analysis provides fast responses, simple operation, and low cost in the detection of CdSe QDs. Here, we report new spectroscopic data about the immobilization on cellulose paper of a Schiff base containing a carboxylic acid substituent, H 2 SB [3]. In addition, we present here our progress in the study of the interaction between H 2 SB and CdSe-Cys QDs. The usefulness of the H 2 SB-modified paper (Figure 1) for the detection of CdSe-Cys QDs was demonstrated by registering an absorbance decrease in the band at 270 mm, with a simultaneous increase in the CdSe-Cys QDs concentration from 100 ppb to 2 ppm [4]. In order to help elucidate how H 2 SB interacts with CdSe-Cys QDs, Cd 2 (SB) 2 (H 2 O) 4 has been synthesized and studied by 1 H NMR, ATR-FTIR, and UV-Vis spectroscopy.

Results and Discussion
The cellulose paper was first primed with (3-aminopropyl)trimethoxysilane (APTMS) after soaking it in a DMF solution for 2 h (Figure 1). In the second reaction step, N-(3-dimethylaminopropyl)-N′-ethylcarbodiimide hydrochloride (EDC·HCl), N-hydroxysuccinimide (NHS), as well as the chemosensor H2SB, were added together to achieve its immobilization on the amine-modified paper (by forming an amide bond) [4].
The observation of three dominant signals located around 270, 330, and 420 nm in the diffuse reflectance spectrum of the chemosensor-modified paper demonstrated its immobilization ( Figure 2). One of the dominant absorption bands appeared to red-shifted around 30 nm, with respect to the spectrum of an ethanol solution of the chemosensor (390 nm). View of the diffuse reflectance spectrum of H2SB-modified cellulose paper previously (green) and after (pale green) the interaction with CdSe-Cys QDs. The diffuse reflectance spectrum of cellulose paper (grey) and the UV-Vis spectrum of H2SB in water solution (blue) have been included for comparison.
The chemosensor-modified cellulose paper after being soaked in CdSe-Cys QDs water solutions for two hours exhibited a decreased absorbance of each band of its diffuse reflectance spectrum. As the most pronounced decrease was observed for the band at 270 nm, this wavelength was chosen to carry out the measurements (Figure 3). The limits of detection (LODs) and of quantification (LOQs) of H2SB were expressed as LOD = 3SD/M and LOQ = 10SD/M, where SD is the standard response deviation and M is the slope of

Results and Discussion
The cellulose paper was first primed with (3-aminopropyl)trimethoxysilane (APTMS) after soaking it in a DMF solution for 2 h (Figure 1). In the second reaction step, N-(3dimethylaminopropyl)-N -ethylcarbodiimide hydrochloride (EDC·HCl), N-hydroxysuccinimide (NHS), as well as the chemosensor H 2 SB, were added together to achieve its immobilization on the amine-modified paper (by forming an amide bond) [4].
The observation of three dominant signals located around 270, 330, and 420 nm in the diffuse reflectance spectrum of the chemosensor-modified paper demonstrated its immobilization ( Figure 2). One of the dominant absorption bands appeared to red-shifted around 30 nm, with respect to the spectrum of an ethanol solution of the chemosensor (390 nm).

Results and Discussion
The cellulose paper was first primed with (3-aminopropyl)trimethoxysilane (APTMS) after soaking it in a DMF solution for 2 h (Figure 1). In the second reaction step, N-(3-dimethylaminopropyl)-N′-ethylcarbodiimide hydrochloride (EDC·HCl), N-hydroxysuccinimide (NHS), as well as the chemosensor H2SB, were added together to achieve its immobilization on the amine-modified paper (by forming an amide bond) [4].
The observation of three dominant signals located around 270, 330, and 420 nm in the diffuse reflectance spectrum of the chemosensor-modified paper demonstrated its immobilization ( Figure 2). One of the dominant absorption bands appeared to red-shifted around 30 nm, with respect to the spectrum of an ethanol solution of the chemosensor (390 nm). View of the diffuse reflectance spectrum of H2SB-modified cellulose paper previously (green) and after (pale green) the interaction with CdSe-Cys QDs. The diffuse reflectance spectrum of cellulose paper (grey) and the UV-Vis spectrum of H2SB in water solution (blue) have been included for comparison.
The chemosensor-modified cellulose paper after being soaked in CdSe-Cys QDs water solutions for two hours exhibited a decreased absorbance of each band of its diffuse reflectance spectrum. As the most pronounced decrease was observed for the band at 270 nm, this wavelength was chosen to carry out the measurements ( Figure 3). The limits of detection (LODs) and of quantification (LOQs) of H2SB were expressed as LOD = 3SD/M and LOQ = 10SD/M, where SD is the standard response deviation and M is the slope of Figure 2. View of the diffuse reflectance spectrum of H 2 SB-modified cellulose paper previously (green) and after (pale green) the interaction with CdSe-Cys QDs. The diffuse reflectance spectrum of cellulose paper (grey) and the UV-Vis spectrum of H 2 SB in water solution (blue) have been included for comparison.
The chemosensor-modified cellulose paper after being soaked in CdSe-Cys QDs water solutions for two hours exhibited a decreased absorbance of each band of its diffuse reflectance spectrum. As the most pronounced decrease was observed for the band at 270 nm, this wavelength was chosen to carry out the measurements ( Figure 3). The limits of detection (LODs) and of quantification (LOQs) of H 2 SB were expressed as LOD = 3SD/M and LOQ = 10SD/M, where SD is the standard response deviation and M is the slope of the calibration curve. The LOD and LOQ calculation results were 245 and 815 ppb, respectively. the calibration curve. The LOD and LOQ calculation results were 245 and 815 ppb, respectively. To study the type of interaction between the CdSe-Cys QDs and H2SB, we obtained Cd2(SB)2(H2O)4·8H2O from the reaction of Cd(OAc)2·2H2O and H2SB in an ethanol solution at room temperature. The similarities between the UV-Vis spectra of Cd2(SB)2(H2O)4·8H2O and CdSe-Cys-H2SB QDs (Figure 4), with three bands at about 220, 250, and 340 nm, evidenced an interaction via metal-ligand coordination between CdSe-Cys QDs and H2SB. The binding constant value of H2SB with Cd 2+ , at room temperature, has been determined from UV-Vis absorption data following the Benesi-Hildebrand equation [5] (Kb = 3.686 × 10 3 M −1 ) for 1:1 (metal:ligand) complexes ( Figure 5). The binding stoichiometry used in the determination of Kb was obtained by elemental analysis. To study the type of interaction between the CdSe-Cys QDs and H 2 SB, we obtained Cd 2 (SB) 2   To study the type of interaction between the CdSe-Cys QDs and H2SB, we obtained Cd2(SB)2(H2O)4·8H2O from the reaction of Cd(OAc)2·2H2O and H2SB in an ethanol solution at room temperature. The similarities between the UV-Vis spectra of Cd2(SB)2(H2O)4·8H2O and CdSe-Cys-H2SB QDs (Figure 4), with three bands at about 220, 250, and 340 nm, evidenced an interaction via metal-ligand coordination between CdSe-Cys QDs and H2SB. The binding constant value of H2SB with Cd 2+ , at room temperature, has been determined from UV-Vis absorption data following the Benesi-Hildebrand equation [5] (Kb = 3.686 × 10 3 M −1 ) for 1:1 (metal:ligand) complexes ( Figure 5). The binding stoichiometry used in the determination of Kb was obtained by elemental analysis. The binding constant value of H 2 SB with Cd 2+ , at room temperature, has been determined from UV-Vis absorption data following the Benesi-Hildebrand equation [5] (K b = 3.686 × 10 3 M −1 ) for 1:1 (metal:ligand) complexes ( Figure 5). The binding stoichiometry used in the determination of K b was obtained by elemental analysis. Chem. Proc. 2022, 12, 92 4 of 7 The 1 H NMR spectrum of the cadmium(II) complex revealed the chelating behaviour of the ligand through the Ophenol, Nimine, and Nsulfonamide atoms. Figure 6 shows the absence of the signal corresponding to OHphenol (12.29 ppm) and NHsulfonamide (8.37 ppm) as well as the upfield shift (about 0.3 ppm) of the -CH=N signal which was observed at 8.63 ppm in H3L. This is consistent with the formation of a cadmium complex with a sulfonamide bridge, similar to those reported for complexes of similar Schiff base ligands [6,7]. The 1 H NMR spectrum of CdSe-Cys-H3L QDs (Figure 7) showed the absence of the signals corresponding to OHphenol and NHsulphonamide, which indicates that the interaction between the QDs and the ligand occurs through its bideprotonated form. The obvious similarities between the 1 H NMR spectra of CdSe-Cys-H2SB QDs and Cd2(SB)2(H2O)4·8H2O (Figure 8) support that the interaction between CdSe-Cys QDs surface and H2SB occurred via metalligand coordination. It must be noted that the differences in chemical shifts of CdSe-Cys-H2SB QDs and Cd2(SB)2(H2O)4·8H2O are due to the solvents used in each case, methanol-d4/D2O and dmso-d6, respectively. The reason for this is the very different solubility of CdSe-Cys-H2SB QDs and Cd2(SB)2(H2O)4·8H2O.  The 1 H NMR spectrum of the cadmium(II) complex revealed the chelating behaviour of the ligand through the O phenol , N imine , and N sulfonamide atoms. Figure 6 shows the absence of the signal corresponding to OH phenol (12.29 ppm) and NH sulfonamide (8.37 ppm) as well as the upfield shift (about 0.3 ppm) of the -CH=N signal which was observed at 8.63 ppm in H 3 L. This is consistent with the formation of a cadmium complex with a sulfonamide bridge, similar to those reported for complexes of similar Schiff base ligands [6,7]. The 1 H NMR spectrum of CdSe-Cys-H 3 L QDs (Figure 7) showed the absence of the signals corresponding to OH phenol and NH sulphonamide , which indicates that the interaction between the QDs and the ligand occurs through its bideprotonated form. The obvious similarities between the 1 H NMR spectra of CdSe-Cys-H 2 SB QDs and Cd 2 (SB) 2  The 1 H NMR spectrum of the cadmium(II) complex revealed the chelating behaviour of the ligand through the Ophenol, Nimine, and Nsulfonamide atoms. Figure 6 shows the absence of the signal corresponding to OHphenol (12.29 ppm) and NHsulfonamide (8.37 ppm) as well as the upfield shift (about 0.3 ppm) of the -CH=N signal which was observed at 8.63 ppm in H3L. This is consistent with the formation of a cadmium complex with a sulfonamide bridge, similar to those reported for complexes of similar Schiff base ligands [6,7]. The 1 H NMR spectrum of CdSe-Cys-H3L QDs (Figure 7) showed the absence of the signals corresponding to OHphenol and NHsulphonamide, which indicates that the interaction between the QDs and the ligand occurs through its bideprotonated form. The obvious similarities between the 1 H NMR spectra of CdSe-Cys-H2SB QDs and Cd2(SB)2(H2O)4·8H2O (Figure 8) support that the interaction between CdSe-Cys QDs surface and H2SB occurred via metalligand coordination. It must be noted that the differences in chemical shifts of CdSe-Cys-H2SB QDs and Cd2(SB)2(H2O)4·8H2O are due to the solvents used in each case, methanol-d4/D2O and dmso-d6, respectively. The reason for this is the very different solubility of CdSe-Cys-H2SB QDs and Cd2(SB)2(H2O)4·8H2O.   The IR spectrum of Cd2(SB)2(H2O)4·8H2O showed a broad band centred at about 3360 cm −1 , attributable to ν(OH), which evidences the hydration of the obtained complex ( Figure  8). The observation of two bands attributable to νas(COO − ) and νs(COO − ) at ca. 1574 and 1393 cm-1 , respectively, seems to indicate the deprotonation of the carboxylic groups of the Schiff base in its complex. The neutrality of this complex is related to the zwitterionic nature adopted by the ligand units, where the carboxylic groups are deprotonated, while their amine N atoms are protonated.
The IR spectrum of CdSe-Cys-H2SB QDs evidences the bideprotonation of the Schiff base ligand, as it showed the absence of bands attributable to ν OH/ν NH modes. Likewise, the presence of two bands attributable to νas(COO − ) and νs(COO − ), at about 1574 and 1391 cm −1 , respectively, evidences the deprotonation of the carboxyl group in the ligand, as it is indicative of the formation of the carboxylate sodium salt. In addition, the clear presence of a strong sharp band (ca. 1615 cm −1 ) is attributed to the C=N group of the ligand. The similarity between the spectra of the ATR-IR spectra of CdSe-Cys-H2SB QDs with Cd2(SB)2(H2O)4·8H2O (Figure 8) supports that the interaction between H2SB and CdSe-Cys QD surfaces occurred via metal-ligand coordination through the Nsulfonamide, Nimine, and Ophenol atoms.  The IR spectrum of Cd2(SB)2(H2O)4·8H2O showed a broad band centred at about 3360 cm −1 , attributable to ν(OH), which evidences the hydration of the obtained complex ( Figure  8). The observation of two bands attributable to νas(COO − ) and νs(COO − ) at ca. 1574 and 1393 cm-1 , respectively, seems to indicate the deprotonation of the carboxylic groups of the Schiff base in its complex. The neutrality of this complex is related to the zwitterionic nature adopted by the ligand units, where the carboxylic groups are deprotonated, while their amine N atoms are protonated.
The IR spectrum of CdSe-Cys-H2SB QDs evidences the bideprotonation of the Schiff base ligand, as it showed the absence of bands attributable to ν OH/ν NH modes. Likewise, the presence of two bands attributable to νas(COO − ) and νs(COO − ), at about 1574 and 1391 cm −1 , respectively, evidences the deprotonation of the carboxyl group in the ligand, as it is indicative of the formation of the carboxylate sodium salt. In addition, the clear presence of a strong sharp band (ca. 1615 cm −1 ) is attributed to the C=N group of the ligand. The similarity between the spectra of the ATR-IR spectra of CdSe-Cys-H2SB QDs with Cd2(SB)2(H2O)4·8H2O (Figure 8) supports that the interaction between H2SB and CdSe-Cys QD surfaces occurred via metal-ligand coordination through the Nsulfonamide, Nimine, and Ophenol atoms. The IR spectrum of Cd 2 (SB) 2 (H 2 O) 4 ·8H 2 O showed a broad band centred at about 3360 cm −1 , attributable to ν(OH), which evidences the hydration of the obtained complex ( Figure 8). The observation of two bands attributable to ν as (COO − ) and ν s (COO − ) at ca. 1574 and 1393 cm −1 , respectively, seems to indicate the deprotonation of the carboxylic groups of the Schiff base in its complex. The neutrality of this complex is related to the zwitterionic nature adopted by the ligand units, where the carboxylic groups are deprotonated, while their amine N atoms are protonated.
The IR spectrum of CdSe-Cys-H 2 SB QDs evidences the bideprotonation of the Schiff base ligand, as it showed the absence of bands attributable to ν OH/ν NH modes. Likewise, the presence of two bands attributable to ν as (COO − ) and ν s (COO − ), at about 1574 and 1391 cm −1 , respectively, evidences the deprotonation of the carboxyl group in the ligand, as it is indicative of the formation of the carboxylate sodium salt. In addition, the clear presence of a strong sharp band (ca. 1615 cm −1 ) is attributed to the C=N group of the ligand. The similarity between the spectra of the ATR-IR spectra of CdSe-Cys-H 2 SB QDs with Cd 2 (SB) 2 (H 2 O) 4 ·8H 2 O (Figure 8) supports that the interaction between H 2 SB and CdSe-Cys QD surfaces occurred via metal-ligand coordination through the N sulfonamide , N imine , and O phenol atoms.