The poor extraction efficiency of the small molecules triggered us to use the target polymers for the transfer of NPs from water to organic layer. The experimental protocol followed was similar to that of the small molecules. Solution of citrate capped Au or Ag NPs (4 mL, 10-4 M) were extracted separately using DCM solution (4 mL) of the target polymers (1 mg). Concentration of the remaining NPs in water layer after extraction (Figure 4.6) was determined by matching its absorbance against a pre- determined calibration curve. Extraction efficiencies for them are enlisted in Table 4.4.
Figure 4.6. Absorption spectra of aqueous layer of (a) Ag and (b) Au NPs before extraction (1) and after extraction with TM6 (2), TM7 (3), POLY9 (4), POLY7 (5) and POLY8 (6).
POLY7 and POLY8 showed excellent extraction efficiencies for both Au and Ag NP (∼
99 %) as compared to POLY9 (∼ 80 %). A positive correlation between the extraction efficiency and the chain length of the polymers was observed. Longer the chain length, greater is the number of binding sites along the polymer backbone, which ensures strong interaction between NPs and polymers and results in enhanced extraction efficiency. In this section, all further discussion will be based on the results obtained with POLY7 because of its highest extraction efficiency. Two control experiments were performed where same amount of NP solutions were extracted with 4 mL of DCM in absence of any of the target compounds and no transfer of NPs to the DCM layer was observed (Figure 4.7a, vial 1 and 3). Thus, the results of the control experiments clearly prove the inertness of DCM towards extraction and confirm the synthesized compounds to be solely responsible for binding to NPs and extraction.
300 400 500 600 700 800
0.0 0.5 1.0
5 6 4 3 2
Absorbance
Wavelength (nm) 1
300 400 500 600 700 800
0.0 0.5
5 6 4 3 2
Absorbance
Wavelength (nm)
a 1 b
Table 4.4. Extraction efficiency of the target compounds (TM6 – TM7 and POLY7 – POLY9) for Au and Ag NPs.
Ligand Efficiency for AgNP
extraction (%)a Efficiency for AuNP extraction (%)a
TM6 18 18
TM7 52 57
POLY7 99 99
POLY8 99 98
POLY9 83 80
aMean value obtained after performing the experiments in triplicate.
Emission intensities of the DCM solution of POLY7 before and after extraction were used as a probe to monitor the binding of NPs to the synthesized compounds (Figure 4.7b). Significant quenching of the polymeric fluorescence was observed after extraction, which indicates strong interaction between NPs and the polymer. The quenching effect of AuNPs was found to be more as compared to that of AgNPs. This may be explained by the smaller size and larger surface area of AuNPs, which help more number of polymer chains to bind on the NP surface and leaves fewer numbers of free molecules.20 Thus the net fluorescence intensity, contributed by the free POLY7, decreases.
TEM images (Figure 4.7c, 4.7d) of the DCM layer after extraction clearly identified the presence of Au and Ag NPs in it and thus proved successful transfer of the NPs to the organic layer without noticeable aggregation. The TEM images of corresponding NPs before extraction have been shown in the inset. ICP-OES analyses performed on the DCM layer after extraction gave additional support to the TEM data by detecting gold and silver.
Figure 4.7. (a) Pictorial representation of the vials after extraction. Vials 1 and 2 contain AgNP, vials 3 and 4 contain AuNP. Extraction of the NP solution in presence of POLY7 resulted in complete transfer of AgNP (vial 2) and AuNP (vial 4) from water (top) to DCM (bottom) layer. Shaking of the NP solutions with DCM alone did not result any phase transfer of NPs (vial 1 and 3). (b) Fluorescence spectra of the DCM layer of vial 2 (○) and vial 4 (●) showed quenching of fluorescence intensity as compared to the DCM solution of POLY7 before extraction (★). TEM images of the dropcasted film of DCM layer of vial 2 (c) and vial 4 (d) confirm transfer of AgNP and AuNP from water to DCM layer. Insets show the TEM images of the corresponding NPs before extraction.
IR spectra and zeta potential of the NPs were recorded before and after extraction to understand the mechanism of phase transfer. The NPs after extraction was purified by centrifugation of the DCM layer (9000 rpm, 5 min) to remove the unbound polymers.
The residue obtained was dried and mixed with solid KBr for recording IR spectra. The dried residue was re-dispersed in ethanol for analyzing zeta potentials. Results obtained with POLY7 are described here. Zeta potentials of the NPs were found (Figure 4.8 and Table 4.5) to change from negative to positive values before and after extraction which indicates change of capping environment of the NPs during extraction.39-42 Ionization of basic groups present on the surface of NPs is known to generate positive surface charge.43 Thus, it is conceivable that the thiophenol groups hanging from polymer chain are responsible for the observed positive zeta potential of POLY7 wrapped NPs. The existence of thiophenol can be explained by the dynamic equilibrium between NPs and binding ligands i.e. breaking and making of PhS-NP bond.
Figure 4.8. Zeta potential distribution of citrate capped AuNP (a) citrate capped AgNP (b) POLY7 capped AuNP (c) POLY7 capped AgNP (d) and only POLY7 (e).
Table 4.5. Zeta potentials of NPs before and after extraction.
Graph NP Capping agent Solvent ξ-potential (mV)
a AuNP citrate Ethanol -21.9
c AuNP POLY7 Ethanol 20.0
b AgNP citrate Ethanol -23.8
d AgNP POLY7 Ethanol 23.4
e - POLY7 Ethanol -1.49
IR spectra of the NPs (Figure 4.9) after extraction resemble that of POLY7 with complete disappearance of the characteristic stretching peaks of citrate at 1597 and 1397 cm-1.39 It further supports the exchange of capping ligands during extraction.
Figure 4.9. IR spectra of (a) Ag and (b) Au NPs before and after extraction. IR of pure POLY7 is included for comparison. Disappearance of characteristic peaks of citrate (1597 and 1397 cm-1) and appearance of peaks of POLY7 in Ag-POLY7 or Au-POLY7 indicate the replacement of citrate by POLY7 during extraction.
3500 3000 2500 2000 1500 1000
Wavenumber (cm-1)
POLY7
% Transmittance
Au-POLY7 Au-citrate
3500 3000 2500 2000 1500 1000
Wavenumber (cm-1)
POLY7
% Transmittance
Ag-POLY7
Ag-citrate a b
4.3.5.2.2. Extraction Using Polymer Coated Electrospun PVA NF
Once the binding ability and ligand exchange efficiency of the target polymers were confirmed, attempts were made to develop a more convenient extraction set-up. Use of electrospun nanofibers (NF) is a promising alternative owing to its high surface area.
NF made of polyvinyl alcohol (PVA NF) has been attempted for this purpose.
Preparation of PVA NF and extraction method has been described in the experimental section. Initially, PVA NF was prepared via electrospinning PVA solution with different percentage of loading of target compounds. But unfortunately, the resulting NF did not show any extraction efficiency, which may be due to the unavailability of the binding atoms on the surface of the NF.
In the next attempt, electrospun PVA NF were coated with target compound solution and were directly immersed in NP solution. Detailed protocol has been provided in the experimental section. Only POLY7 and POLY8 were engaged in this study whereas, POLY9 were excluded due to its poor extraction efficiency observed in the case of liquid phase extraction. UV spectra of the water layer were recorded before and after extraction (Figure 4.10a, 4.10b). The results showed similar extraction efficiency of POLY7 and POLY8 coated PVA NF for both Ag and Au NPs as in the case of liquid phase extraction. FESEM images of the used NF mat clearly showed the attachment of NPs on the surface (Figure 4.10c, 4.10d), which was further supported by EDX measurements. Extraction efficiency of this technique was found to be sensitive towards the thickness and uniformity of the coating. Use of DCM or toluene as solvent for the polymers resulted in non-uniform coating of the NF, which in turn hinders sufficient
interaction between the polymers and NPs and results in inefficient extraction. Acetone was chosen as the optimum solvent after screening through a list of solvents.
Figure 4.10. Absorption spectra of water layer before and after removal of (a) Ag and (b) Au NPs. (★) NP solution before extraction, (☆) NP solution after extraction with POLY7 coated PVA NF, (○) NP solution after extraction with POLY8 coated PVA NF.
SEM images of POLY7 coated PVA NF after extraction show attachment of (c) AgNP and (d) AuNP onto the fiber. Insets show SEM images of the corresponding PVA NF before extraction.