N-GQDs as SERS Substrate

Chapter 3: Origin of High Photoluminescence Yield and High SERS Sensitivity of Nitrogen-

3.3. Results and Discussion

3.3.4. Application of N-GQDs

3.3.4.1. N-GQDs as SERS Substrate

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RhB at ~1648 cm^-1, 1507 cm^-1, 1359 cm^-1, 1188 cm^-1, and 766 cm^-1 are observed on N-GQDs substrate, and a tiny signal appears at ~1648 cm^-1 and 1507 cm^-1 with U-GQDs substrate, while S-GQDsdo not show any measurable SERS signal. Different Raman modes of bare RhB (10^-2 M) on Si substrate are shown in Fig. 3.10(b). Based on the high SERS signal, N-GQDs coated on Si substrate are optimized with different laser excitation for better result. Fig. 3.10(c) shows the Raman signal of RhB (10^-4 M) at laser excitations 488 nm, 514 nm, and 633 nm, demonstrating

Fig. 3.10. (a) Raman spectra of RhB (10^-4 M) using U-GQDs, N-GQDs, and S-GQDs as SERS substrates with laser excitation 514 nm, where ‘*’ symbol indicates the SERS enhanced peak of RhB at 1648 cm^-1. (b) Raman spectrum of RhB (concentration 10^-2 M) with different Raman active modes under laser excitation 488 nm. (c) SERS effect at different laser excitation wavelengths for 10^-4 M RhB on N-GQDs substrate.

the highest enhancement of the SERS signal with 488 nm laser excitation. As a reference at 488 nm laser excitation, the enhancement factor (EF) for N-GQDs is calculated with the following equation:

𝐸𝐹 = ^{𝐼𝑆𝐸𝑅𝑆}_𝐼

𝑅 ×_𝐶^𝐶𝑅

𝑆𝐸𝑅𝑆×_𝑃 ^𝑃×𝑡

𝑆𝐸𝑅𝑆𝑡_{𝑆𝐸𝑅𝑆} (3.3.1)

where, ISERS and IR are the Raman intensity at ~1648 cm^-1 of RhB with and without the presence of N-GQDs at the known concentration of RhB CSERS and CR, respectively. ‘P’ represents the laser power, and ‘t’ is the accumulation time. For the present case, ‘P’ and ‘t’ are the same for Raman and SERS measurement. As the SERS process depends on the effective number of the adsorbed analyte molecules, the eqn. (3.3.1) is modified as

𝐸𝐹 = ^{𝐼𝑆𝐸𝑅𝑆}_𝐼

𝑅 ×_𝑁^𝑁𝑅

𝑆𝐸𝑅𝑆 (3.3.2)

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where NR is the number of RhB molecules contributing to bulk Raman signal, and NSERS is the number of absorbed RhB molecules contributing to SERS Raman signal. The number of RhB molecules is estimated as:

𝑁 =_𝐴^{𝐴𝑙𝑎𝑠𝑒𝑟}

𝑠𝑢𝑏𝑠𝑡𝑟𝑎𝑡𝑒× 𝑀𝑉𝑁_𝐴 (3.3.3)

Alaser is the area of the laser spot, and Asubstrate is that of the substrate. ‘V’ is the volume of the RhB solution, and ‘M’ is the molar concentration. For our case, the laser spot (1 m diameter) and the size of the substrate (0.5cm × 0.5 cm) are identical for SERS and Raman measurements. For both measurements, the volume of the RhB solution is used as 20 L. Assuming the uniform distribution of the RhB solution on the substrate,

_𝑁^𝑁𝑅

𝑆𝐸𝑅𝑆= _𝑀^𝑀𝑅

𝑆𝐸𝑅𝑆 (3.3.4)

where, MR andMSERS are the molar concentration of RhB for Raman and SERS measurement, respectively. For the calculation of EF in the present case, the values of MR and MSERS are 10^-2 M and 10^-4 M, respectively. Thus,

𝐸𝐹 = ^{𝐼𝑆𝐸𝑅𝑆}_𝐼

𝑅 ×_𝑀^𝑀𝑅

𝑆𝐸𝑅𝑆 (3.3.5)

With ISERS/IR ~3.2at ~1648 cm^-1, the value of EF is obtained as ~3.2×10³ for 488 nm excitation.

A comparison of the SERS performance of different carbon-based substrates with the present work is presented in Table 3.4. Thus, the enhancement factor for this N-GQDs substrate is the highest among the reported values without plasmonic components.

Table 3.4. Comparison of SERS performance of different carbon-based substrates with the present work.

SERS Substrate

Laser excitation

(nm)

Value of EF

Mechanism of Enhancement

LOD

(nM) Ref.

GQDs 532 4.3 10² CM 1 ³

GQD assembled

Nanotubes 633 6.9 10¹ CM 1 ²³

Oxidized graphene 514 1.7 10³ CM - ²⁰

Au NP–graphene–Ag NP

hybrid 532 1.0 10⁹ EM & CM 0.0001 ⁴⁴

Ag-RGO 633 1.0 10⁷ EM & CM 0.1 ⁴⁵

N-GQDs 488 3.2 10³ CM and FRET 0.1 This

work

3.3.4.1.1. Mechanism of SERS Enhancement

The SERS enhancement mechanisms are reported mainly to be contributed by the electromagnetic enhancement (EM) and CM factors, where EM arises due to the localized electromagnetic field mainly in the presence of metallic components and CM involves the charge transfer phenomena.

3, 23 For GQDs, large surface area with accessible edges and dangling bonds can lead to effective adsorption of target molecules. In the present case, the possible reasons for the performance of N- GQDs as efficient SERS substrate are as follows. The π−π staking between the graphene and absorbed molecules could shorten the distance between the molecules and the substrate, which could facilitate an efficient charge transfer from substrate to target molecules.⁴⁶ Among all types of synthesized GQDs, the sp² domain in N-GQDs acquires the highest electronic charge contents, which facilitate maximum charge transfer to the target molecules by π−π interaction. Due to the high absorption of RhB, the strongest SERS signal was observed previously with 514 nm laser excitation.⁴⁷ In the present case, 488 nm laser excitation shows higher efficiency than 514 nm excitation for SERS enhancement. This is very likely to be related to the FRET process from N- GQDs to RhB. To clarify the contribution of the FRET process with RhB, the MB target (concentration 10^-3 M) is also tested on the different types of GQDs for the SERS enhancement effect, as shown in Fig. 3.11(a) with 488 nm excitation. Although MB has aromatic structure, a low EF of SERS signal is observed with all kinds of the GQDs substrates, which reveals that the π−π interaction is not the only mechanism for the SERS signal in N-GQDs. Interestingly, the PL emission peak of N-GQDs (at excitation 488 nm) coincides with the absorption peak of RhB, while the absorption of MB is far away, as shown in Fig. 3.11(b). Thus, due to overlap of the emission energy of donor (N-GQDs) with the absorption energy of acceptor molecule (RhB), and their close interaction, FRET is most likely to take place between N-GQDs and RhB. Thus, 488 nm excitation shows the highest EF in this donor-acceptor pair, and the EF mechanism is essentially of chemical nature, i.e., CM type. Note that among differently prepared N-GQDs substrates, N-GQDs prepared with 15 mg/mL GO concentration shows the maximum enhancement (see Fig. 3.11(c)).

Interestingly, the same sample shows the highest PL intensity as well. This reveals the excited- state charge/energy transfer from N-GQDs to target molecules (RhB) is primarily responsible for the SERS enhancement in the present case, and this could be due to the radiative functional groups in N-GQDs. However, the contribution of EM enhancement, if any, cannot be ruled out entirely from the above observation. To ascertain the contribution of the functional groups in the SERS

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enhancement, N-GQDs substrates are subjected to vacuum annealing at different temperatures (~200 °C, 400 °C and 600 °C) for 2 h to systematically control and eliminate only the functional groups.^{37, 48} Remarkably, with increasing annealing temperature of N-GQDs substrate, the EF of 10^-4 M RhB for 1648 cm^-1 peak goes down systematically under 488 nm laser excitation, as shown in Fig. 3.11(d). The inset shows the corresponding SERS spectra on annealed N-GQDs substrates at different temperatures. The reduction of the functional groups from N-GQDs by vacuum annealing reduces the energy transfer from N-GQDs to RhB in the FRET process. Thus, in the

Fig. 3.11. (a) SERS effect with 10^-3 M concentration of MB target using U-GQDs, N-GQDs, and S-GQDs as SERS substrates. ‘#’ indicates the SERS peak of MB being monitored with 488 nm excitation. (b) Normalized PL spectrum of N-GQDs (0.2 mg/mL) with excitation 488 nm and the absorption spectra of RhB and MB with concentration 10^-5 M. The overlap of spectra indicates the possibility of FRET between N-GQDs and RhB. (c) Comparison of SERS enhancement on N-GQDs substrate prepared with different proportions of GO precursor. (d) The change of the EF value as a function of annealing temperatures for N-GQDs substrates; the corresponding Raman spectra are shown in the inset.

present case, the π−π interaction and the functional groups both contribute to the SERS enhancement on N-GQDs substrate with the RhB target. With 633 nm laser excitation (see Fig.

3.10(c)), EF is calculated as ~10² for 10^-4 M concentration of RhB at 1648 cm^-1 peak, which is ~32 times lower than that of the 488 nm excitation. As there is no possibility of FRET between N- GQDs and RhB at 633 nm excitation, the SERS enhancement with 633 nm excitation must be only due to the π−π interaction. Thus, the contribution of FRET to SERS enhancement is only ~32, and that of CM is 10² yielding an overall enhancement of ~3.2x10³. This confirms the combined contribution of CM interaction and FRET for the observed SERS enhancement with N-GQDs. In the case of U-GQDs and S-GQDs, due to the low PL intensity, the FRET process is less efficient, and hence overall SERS enhancement is low. Note that due to the semiconducting nature and spherical shape of GQDs, electromagnetic contribution to the observed SERS in the visible region of wavelength is negligible.⁴⁹

3.3.4.1.2. N-GQDs SERS Substrate as Sensor

Based on the above discussion, the N-GQDs substrate is implemented to detect a very low concentration of RhB in nM level. Fig. 3.12(a) shows the systematic enhancement of the

Fig. 3.12. (a) Concentration-dependent changes in the SERS signal intensity of RhB target on N-GQDs substrate, where ‘*’ symbol indicates the SERS enhanced peak of RhB at 1648 cm^-1. (b) The variation of Raman intensity of 1648 cm^-1 peak with the logarithm of RhB concentration shows a linear variation in the range 10^-10–10^-5 M.

characteristic Raman peaks of RhB with increasing concentration of RhB in the range10^-10–10^-4 M. The variation of Raman peak intensity at ~1648 cm^-1 follows a linear nature with the log of

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RhB concentration, as presented in Fig. 3.12(b). We achieve a detection limit down to 0.1 nM, which is superior to the previous reports with GQDs substrate, as presented in Table 3.4.