Figure 2. The flow chart for synthesis and characterization of nanomaterials.
Methods
Ligand Synthesis and Exchange:
To increase the solubility and enable selective binding ability of the nanomaterials, the lipophilic ligands of our synthesized nanomaterials should be replaced by special ligands through ligand exchange. We used a well-developed procedure (developed in Zhong Lab@ShanghaiTech) to synthesize this ligand. We performed the ligand exchange experiment to replace the origin ligand of the nanomaterials. The result of ligand exchange experiment was shown by the following image. After this experiment, the nanomaterials were ready to be harnessed as the binding material of the biofilm and the solar energy harvester of the Solar Hunter.
Ultraviolet-visible (UV-Vis) spectroscopy:
The Ultraviolet-visible (UV-Vis) spectroscopy is an absorption spectroscopy in the near-UV and visible spectral region. The UV-Vis spectroscopy is normally used as a quantitative and qualitative characterization for different analytes. As QDs and NRs solutions contain large amount of transition metal irons, they exhibit absorption in UV-Vis range due to stimulated transision. And after determined the molar absorptivity(ε) from the concentration of Cd2+, the molarity can be calculated by the following equation:
A=εBC
In the equation, A(Abs) refers to absorption; ε(Abs*cm-1*M-1) refers to molar absorptivity; B(cm) refers to the length of cuvette; C(M) refers to the molarity of the QD or NR solution.The UV-Vis spectrometer we use is Agilent Cary 5000.
Photoluminescence(PL) spectroscopy:
The Photoluminescence(PL) spectroscopy is a widely used tool for characterization of electronic and optical properties of analytes, especially quantum dots and nanorods. Briefly, the PL spectroscopy detects photon emitted in a certain spectral region when the samples are excited by light at a certain wavelength. The PL spectroscopy can be used to determine the excitation peak and emission peak. The position of the emission peak can indicate whether the synthesis of QDs and NRs is successfully performed. The best excitation wavelength and the best emission wavelength, which are of great importance for utilizing the use of QDs and NRs as characterization tools, can also be known from the position of the peaks. The PL spectrometer we use is HORIBA FL-3.
Transmission Electron Microscope(TEM):
Transmission Electron Microscope (TEM) was used to image the shapes and sizes of QDs and NRs. The 120KV TEM (National protein facility center, Shanghai, China) we used provides the resolution of around 0.2nm. The TEM sample of our lipophilic QDs and NRs solution is prepared by simply dropping 10 μL of the solution on to the copper grid.
Results
Synthesis and Characterization of CdS Nanorods
We synthesized CdS nanorods following a procedure adapted from a previously published protocol[1]. The synthesis procedure mainly contains two steps: synthesis of CdS seeds, followed by growth of CdS nanorods using CdS nanoparticles as nuclei.
Figure 3. Solutions of CdS nanoparticle seeds in TOP (left), CdS NRs in toluene (right)
Characterization of UV-Vis was performed to calculate the concentration of CdS seed in TOP solution and CdS nanorod in toluene solution. Also, PL spectrum of CdS NRs in toluene was collected to investigate the emission attribute of the nanorod. TEM image of the nanorods was acquired to study the shape and size distribution.
Figure 4. Result of CdS NRs ligand exchange experiments.
A ligand exchange experiment was performed and the result is shown in Figure 4
Figure 5. UV-Vis spectra of CdS seeds in TOP (A) and CdS nanorods in toluene (B);. Photoluminescence Spectrum of CdS nanorods in toluene (C).
The concentration of CdS seeds and CdS NR products were determined by using the UV-Vis spectrometer (Fig. 5 A, B). The peak shown in PL spectrum (Fig. 5 C) matches the absorption peak of the UV-Vis spectra, which thus proves the synthesis of CdS NRs.
Figure 6. TEM images of CdS NRs (A) and size distribution of CdS NRs (B). Note: 100 NRs in total were measured to determine the size distribution. The NRs thus measured have an average diameter of 3.93±0.57nm and average length of 66.81±6.74nm.
TEM confirms that synthesized products show nanorod feature, with an average diameter of 3.93±0.57nm and average length of 66.81±6.74nm.
Synthesis and Characterization of CdSe QDs
We followed a protocol from a previously published paper[2]. After synthesis, UV-Vis spectroscopy, PL spectroscopy and TEM were carried out to confirm the structures and features of CdSe QDs. The photoluminescence of CdSe QDs can be observed by naked eyes.
Figure 7. Experimental set-up for synthesis of CdSe QDs (a) , synthesized NTA-capped CdSe in PBS solution (b) and PL of CdSe QDs in PBS under UV light (c, excitation wavelength 354 nm)
A ligand exchange experiment was performed and the result is shown in Figure 8.
Figure 8. UV-Vis spectrum of CdSe QDs in CH2Cl2 (a); PL spectrum of CdSe QDs in CH2Cl2 (b)and The TEM image of CdSe QDs(c).
We used UV-Vis spectrum to confirm the successful synthesis of CdSe QDs. The absorption attribute of the QDs has indeed been approved (Fig 8. A). The QDs/CH2Cl2 exhibits an emission peak at 528nm, which matches the absorption peak of the UV-Vis spectra (Fig 8. B). Finally, we used TEM to show that CdSe QDs have nanoparticle features, with an average diameter of 6-8 nm (Fig 8. C).
