Dopamine Sensing with GO/WS 2 Hybrid

Chapter 4: Quantitative Understanding of the Ultra-Sensitive and Selective Detection of

4.3. Results and Discussion

4.3.4. Dopamine Sensing with GO/WS 2 Hybrid

Fig. 4.5. (a) Normalized PL spectra of GO/WS2 hybrid at different pH, and (b) the corresponding variation of the PL peak position with pH. (c) Stability of the GO/WS2 hybrid with time. Optimization of DA sensing parameters with the variation of (d) GO concentration, (e) pH value, and (f) the reaction time for a fixed concentration of WS2 QDs and DA.

97 | 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

Fig. 4.6. (a) PL spectra of GO/WS2 hybrid before and after the addition of different concentrations of DA (10 pM–10

M). (b) The variation of the relative change in the PL intensity (1- I/I0) as the function of DA concentration (Q) is fitted with different models (SV, CT, and SV-CT) in the DA concentration range 1 nM–10 M. For better visualization of the deviation from the well-known SV and CT model, the inset shows the variation of (1- I/I0) vs. log(Q). (c) The linear variation of I0/I with DA concentration of 1–10 nM with LOD 0.3 nM.

dynamic quenching behavior or the ‘Sphere of action’ model discussed earlier. To explain the nature of the quenching in the present case in a wide concentration range of DA (10 pM–10 M), we need to incorporate surface adsorption of DA followed by charge transfer phenomena and ground-state complex formation. Note that in the conventional models of quenching, surface adsorption is not taken into consideration. However, this seems to be obvious and important for the present case, as the defect sites can readily act as the sites for adsorption. For the consideration of ground-state complex formation, the well-known linear Stern-Volmer (SV) equation can be used:

𝐼(𝑄)= ^𝐼0

1+𝐾𝑠𝑣𝑄 (4.2)

where I0 is the initial intensity of PL, and Ksv is a constant. Unfortunately, the experimental data does not follow the linear SV equation, particularly at the higher concentration region of DA, as shown by the dashed line in Fig. 4.6(b) and the corresponding inset. Note that only over a small range of concentration (1–10 nM), a linear variation of I0/I with Q is observed with a limit of detection (LOD) of 0.3 nM, as shown in Fig. 4.6(c). However, over a wide range of concentrations, it is highly non-linear. To account for the nonlinearity, we incorporate the surface adsorption of the quencher and the charge transfer at the interface between two species. To describe the photo- excited charge transfer process in semiconducting materials, the first-order rate equation can be introduced as follows:⁴²

𝑑𝑀

𝑑𝑡 = 𝐺 − [𝐾₁+ 𝐾₂(𝑄)]𝑀 (4.3.1)

Here, ‘M’ is the number of excitons in GO/WS2 hybrid at any time ‘t’. ‘G’ is the generation rate, K1 is the decay rate constant of excitons, ‘Q’ is the concentration of the quencher, and the concentration-dependent decay rate K2(Q) is the rate of charge transfer from an excited state to the quencher (DA). For the case of steady-state (𝑡 → ∞),

𝑀(𝑄) =_𝐾 ^𝐺

1+𝐾₂(𝑄) (4.3.2)

Considering the surface adsorption of DA on GO/WS2, K2(Q) is assumed to follow the relation,^39,

49

𝐾₂(𝑄) = 𝐾₂(0)(1 −_𝛼𝑄+1^𝑆 ) (4.3.3)

where K2(0) is the rate of charge transfer in the absence of DA, ‘S’ is the charge transfer efficiency, and ‘’ is the adsorption probability of DA on GO/WS2 hybrid. By using the value of K2(Q) in eqn. (4.3.2), the PL intensity I(Q) being proportional to M(Q) is expressed as:

𝐼(𝑄) = 𝑃𝑀(𝑄) =

𝑃𝐺 𝐾1+𝐾2(0) 1−( ^𝐾2(0)𝑆

𝐾1+𝐾2(0)⁡×_𝛼𝑄+1¹ )

or, 𝐼(𝑄)= ^𝐸

1−⁡_𝛼𝑄+1^𝐹 (4.3.4)

where, 𝐸 =_𝐾 ^𝑃𝐺

1+𝐾₂(0), and 𝐹 =_𝐾^𝐾2(0)𝑆

1+𝐾₂(0) are the constants, ‘P’ is the collection efficiency of PL.

Interestingly, the charge transfer equation seems to be followed quite well in the PL quenching behavior in higher concentration range of DA, as shown in Fig. 4.6(b). Thus, for a comprehensive understanding of the quenching process, the combined equation of the linear SV eqn. (4.2) and the charge transfer (CT) equation eqn. (4.3.4) need to be considered as follows:

𝐼(𝑄)= ^𝐼0

1+𝐵𝑄+ ^𝐸

1−⁡_𝛼𝑄+1^𝐹 (4.4)

where, ‘B’, ‘E’ and ‘F’ are the constants. This combined model is termed as SV-CT model and at Q =0, right-hand side of eqn. (4.4) is taken as I0. Interestingly, the variation of (1- I/I0) vs. Q nicely follows the SV-CT model in the wide concentration region (1 nM–10 M), as shown in Fig. 4.6(b).

The inset of Fig. 4.6(b) shows the concentration data in the logarithmic scale, indicating a strong deviation of the SV and the CT model from the experimental data. In contrast, our data follows the SV-CT model of PL quenching over the entire range of concentration. Additionally, the value of fitting parameter  =0.7 confirms the high adsorption probability of DA on GO/WS2. Thus, it can be concluded that the ground state complex formation followed by charge transfer from WS2

99 | 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

QDs to GO and then to DA leads to unusual quenching of PL in the GO/WS2 hybrid. It is evident from the experimental data that using the newly developed DA sensor, we able to detect as low as 10 pM concentration of DA. Note that a comparative analysis of the reported works and this work is presented in Table 4.1, which shows that the GO/WS2 hybrid system achieves ~four orders of Table 4.1. Comparative performance of fluorescence-based dopamine sensors using WS2 QDs and GO based 2D materials.

magnitude higher sensitivity than the reported systems for DA sensing. The change of the PL intensity of bare WS2 QDs is shown in Fig. 4.7(a) with DA concentration 0.1–10 M. Further, for the comparative study of the quenching efficiency of various systems, the PL spectra of bare WS2

QDs and GO/WS2 are shown before and after the addition of 10 M DA under identical conditions (see Fig. 4.7(b)). Interestingly, for the WS2 QDs alone, barely ~35% quenching is achieved with DA, while for the GO/WS2 hybrid system, ~87% quenching is observed after the addition of 10

M DA. A comparative study of the PL quenching of WS2 QDs hybrid with U-GQDs and S-GQDs in the presence of DA is shown in Fig. 4.7(c, d). With U-GQDs or S-GQDs as mediators, the quenching efficiency is much smaller (~29% and 4%) than that of GO. Thus, GO plays a crucial role in the ultrasensitive detection of DA.

System Detection Range LOD Ref.

WS2 QDs 5.0 – 50 M 3.3 M ⁴

WS2 QDs/Au⁺³ 240 nM – 0.78 M 230 nM ¹⁴

Fluorescent GO 250 nM – 20 M 94 nM ¹⁵

WS2 QDs 100 nM – 10M 100 nM This work

GO/WS2 QDs 10 pM – 10M 10 pM This work

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.