Chapter 4: Quantitative Understanding of the Ultra-Sensitive and Selective Detection of
4.3. Results and Discussion
4.3.5. Mechanism of Superior Dopamine Sensing with GO/WS 2 Hybrid….… 100
Fig. 4.7. (a) The change of the PL intensity of bare WS2 QDs in the presence of a different concentration of DA.
Comparison of PL quenching efficiency of (b) GO, (c) U-GQDs, and (d) S-GQDs with a fixed concentration of WS2
QDs and 10 M DA.
101 | D o p a m i n e S e n s i n g a t p M L e v e l u s i n g G O / W S2Q D s H y b r i d
than that of S (2.51),40 after the interaction of WS2 with the oxygen functional groups of GO, the electronic charge increases around O, and subsequently, in the oxygen functional groups of GO.
Fig. 4.8. (a) Comparison of the high-resolution XPS spectra of O 1s in GO, GO/WS2, and GO/WS2/DA hybrids. Each spectrum is fitted with a Shirley baseline. (b) Deconvolution of the Raman spectra of GO, GO/WS2, and GO/WS2/DA in the frequency range of ~1050–1700 cm-1. In each case, the symbols denote the experimental data, and the solid lines show the fitted curves. The respective peak positions in different samples are indicated by the vertical dashed lines. (c) Comparison of the Raman spectra of WS2 and GO/WS2 hybrids showing a gradual blue shift of the A1g
Raman mode of WS2 QDs with the addition of GO, GO/DA, respectively, while the E2g peak remains unchanged (excitation: 488 nm).
This difference in electronegativity essentially allows the charge transfer from WS2 to GO. In Fig.
4.8(b, c), the Raman spectra of GO, WS2, GO/WS2, and GO/WS2/DA are deconvoluted for a better understanding of the complex formation and charge transfer processes. The fitting parameters are Table 4.2. Details of the deconvoluted high-resolution XPS spectra of O 1s for GO, GO/WS2, and GO/WS2/DA.
Sample Peak position (eV) Peak identity % of contributions
GO
530.6 COOH 12
532.2 C=O 55
533.4 C-OH/C-O-C 33
GO/WS2
530.5 COOH 12
532.0 C=O 66
533.0 C-OH/C-O-C 22
GO/WS2/DA
530.4 COOH 8
531.8 C=O 81
532.9 C-OH/C-O-C 11
tabulated in Table 4.3. In Fig. 4.8(b), the deconvoluted Raman spectra show the presence of D, M, G, and D' bands. M and D' band are assigned to the metallic contribution and structural disorder/edges states in GO, respectively.30, 46 As compared to GO, GO/WS2 hybrid shows a redshift of G peak by ~11 cm-1, while other peak positions remain almost unchanged. This is indicative of the charge transfer from WS2 to GO, which enhances the overall electronic charge in Table 4.3. Details of the Raman modes for GO, GO/WS2, and GO/WS2/DA.
the graphitic domain.42 On the other hand, the A1g peak of WS2 QDs shows a blueshift of ~0.2 cm-
1 in the GO/WS2 hybrid (see Fig. 4.8(c), while the E2g mode remains unchanged, which is the indication of the charge deficiency in WS2 QDs, after its interaction with the GO.17 Further, the unchanged position of E2g mode confirms that GO interacts with WS2 QDs mainly in the out-of- plane direction. The charge transfer from the WS2 QDs to GO is also energetically favorable considering the relative band gaps of the two components. Since WS2 QDs have a high bandgap of ~4.08 eV, while the GO has a lower bandgap (~3.75 eV), the electrons can be easily transferred from WS2 QDs to GO, which is consistent with the Raman analysis. Next, we consider the interaction of DA with GO/WS2 hybrid. It is well known that DA has an aromatic structure with two catechol groups. Due to the aromatic structure of DA, it can easily attach with GO by the
interactions.1 Ren et al. reported strong adsorption of DA on GO surface by the formation of strong hydrogen bonding between GO and DA, in addition to non-covalent interaction in the aqueous medium.24 It is well known that in the basic medium, DA is oxidized to dopamine-o-quinone (DQ) by the conversion of –OH groups of DA to C=O groups.1, 47 As we have observed a higher quenching efficiency at higher pH (see Fig. 4.5(e)), the interaction of GO and DA via hydrogen bond formation is very unlikely in the basic medium. Note that after the interaction of DA with
Sample Raman bands (cm-1)
D M G D' E2g A1g
GO 1356 1528 1586 1612 - -
WS2 QD - - - - 357.3 421.4
GO/WS2 1356 1529 1577 1610 357.3 421.6
GO/WS2/DA 1356 1531 1582 1611 357.3 422.0
Peak identity Defects (GO)
Metallic (GO)
In-plane vibration of
C=C bond (GO)
Structural disorder
/edges (GO)
In-plane vibration of
W-S bond (WS2)
Out-of-plane vibration of W-
S bond (WS2)
103 | D o p a m i n e S e n s i n g a t p M L e v e l u s i n g G O / W S2Q D s H y b r i d
GO/WS2 hybrid, the O 1s XPS spectrum of the composite reveals a higher concentration of C=O, confirming the oxidation of catechol groups of DA (see Fig. 4.8(a) and Table 4.3). Remarkably, the adsorption of DA molecules into the GO surface is observed to follow the Freundlich isotherm, as shown in Fig. 4.9(a). Freundlich isotherm is expressed as:15, 45
𝑙𝑜𝑔[𝑄]𝑑𝐼 = 𝑚𝑙𝑜𝑔𝑑𝐼[𝑄]
𝑚𝑎𝑥+ 𝑐 (4.5)
where dI is the change of the PL intensity of GO/WS2 at PL maxima, dImax is the change of the PL
Fig. 4.9. (a) Freundlich isotherm model fitting for the multilayer adsorption of DA on GO sheets. (b) The change of the absorption spectra of WS2 QDs after the composite formation with GO and DA. (c) An illustration of the energy band diagram of GO/WS2 hybrid and the charge transfer process from WS2 QDs to DA via GO. The horizontal dashed lines indicate defect states in WS2 QDs and GO. (d) Time-resolved PL spectra of WS2 QDs, GO/WS2, and GO/WS2/DA; the corresponding average decay times are shown in the inset.
intensity at saturation, ‘m’ and ‘c’ are the constants. Thus, in the present case, the multilayer adsorption of DA occurs on the GO/WS2 hybrid platform. Note that a higher absorbance of the GO/WS2/DA system shown in Fig. 4.9(b) is consistent with the above. In the present case, as the non-covalent interaction between GO and DA is more likely than the interaction between WS2
QDs and DA, the absorption is mainly governed by the adsorption of DA on the GO surfaces.
Moreover, DQ being a strong electron acceptor by nature,17, 48 there is a possibility of charge transfer from GO to DA. Xu et al. described DA as a reducing agent for the preparation of reduced GO from GO.49 From Fig. 4.8(a), the reduction of the contribution of COOH functional groups from ~12% to 8% in the presence of DA in the O 1s spectrum confirms the same. Punckt et al.
demonstrated the high electrical conductivity of reduced GO.50 Thus, in the presence of DA, the reduction of GO also facilitates the charge transportation from WS2 to DA via GO sheets. A schematic of the charge transfer process from WS2 QDs to DA through GO is illustrated in Fig.
4.9(c). For the quantitative analysis of the charge transfer process, the TRPL spectra of WS2 QDs are analyzed before and after the formation of various hybrids. In Fig. 4.9(d), it is observed that the change of the lifetime (avg ~8.8 ns) of WS2 QDs is only ~0.6 ns after GO/WS2 hybrid formation, while with the addition of DA, the composites exhibit very fast decay (avg ~4.6 ns).
Note that a negligible change of avg for the WS2/DA system discards the possibility of direct charge transfer from WS2 QDs to DA. Thus, the excited state charge transfer from WS2 QDs to DA through the GO layer causes a strong PL quenching. In presence of DA, the charge transfer phenomenon is consistent with the Raman spectral analysis, shown in Fig. 4.8(b, c). In Fig. 4.8(b), the G peak of GO/WS2/DA blue shifts by ~5 cm-1 as compared to that of GO/WS2, confirming the reduction of electronic charge in the graphitic domain of GO due to its interaction with DA.
Additionally, the gradual blue shift of the A1g peak of WS2 QDs by ~0.2 cm-1 and 0.6 cm-1 in GO/WS2 and GO/WS2/DA, respectively, is consistent with the charge transfer from WS2 QDs to GO to DA. The excited states charge transfer from WS2 QDs to GO increases the electronic charge in GO, which produces a strong in-plane vibration of sp2 bonded carbon of GO, and after the attachment of DA with GO via interactions, the strong electron acceptor DQ takes out the electrons from GO. As a result, in the presence of DA, the electronic charge density in GO is reduced. Note that the direct charge transfer from WS2 to DA is insignificant compared to the charge transfer via GO, as we have observed from the PL quenching efficiency in Fig. 4.7(a, b).
As compared to bare WS2 QDs, GO/WS2 hybrid causes very efficient charge separation and charge
105 | D o p a m i n e S e n s i n g a t p M L e v e l u s i n g G O / W S2Q D s H y b r i d
transfer to DA due to the interaction of DA and GO, resulting in the ultra-high sensitive detection of DA. Note that with U-GQDs or S-GQDs mediator, we have observed negligible quenching of WS2 QDs PL intensity in the presence of DA. Due to the presence of abundant functional groups and small size of U-GQDs and S-GQDs, their conductivity is very low. Thus, in the present case, GO not only helps in the interaction with DA but also behaves like a good charge transport medium from WS2 QDs to DA and thus leads to an efficient quenching of PL intensity, finally leading to the ultrasensitive detection of DA.
4.3.6. Selectivity in Dopamine Sensing
Selectivity is a very crucial parameter in sensing performance. For the use of GO/WS2 hybrid as an efficient DA sensor in the real samples, the interference species, such as organic compounds, metal ions, solvents, should be tested under identical experimental conditions. In the present case, glucose (GL), uric acid (UA), ascorbic acid (AA), L-cysteine (L-Cys), glycine (Gly), and thiourea (thio) at a concentration of 10 M are used as organic compounds, and Na+, Cu2+, Ca2+, Al3+, Cd2+, Hg2+, Pb2+ are used as metal ions. H2SO4, HCl, DMF, and DMSO are used as the interfering solvents. Fig. 4.10(a) represents the relative change of the PL intensity (I0/I) of GO/WS2 hybrid with the addition of different species. Interestingly, a strong quenching of PL intensity is observed only with the presence of DA, and no significant quenching is found by the addition of other solvents/samples. Here, the interaction between DA and GO helps to select only DA among all other interfering agents. Thus, the GO/WS2 hybrid system performs as a highly sensitive and selective sensor of DA.
Fig. 4.10. (a) The selectivity of DA sensing by GO/WS2 hybrid in the presence of different interfering species (concentration 10M). (b, c) The evolution of PL intensity of GO/WS2 with varying concentrations of spiked DA in the Brahmaputra River water and the human blood serum, respectively. The corresponding inset shows the relative change of the PL intensity (1-I/I0) with spiked DA concentrations.