Reading and Glass Slide Preparation Control Experiment

When I first found out that I would be doing physical chemistry research over the summer, I had no idea how much actual reading was involved. Out of the three weeks that I’ve been here, I would say that about 75% of the time I was scanning through stacks on stacks of articles and googling words that stumped me. This is not necessarily a negative thing because I have learned so much background information on my project that has given me a better appreciation of the nuances involved in photovoltaic research. However, if I have to read the words localized surface plasmon resonance (LSPR), quantum yield, radiative decay rate, or extinction spectra again anytime soon I might go crazy. The fact that we digest so much literature in the Wustholz lab is not because we have nothing important to do but because we currently only have 1 laser and confocal microscope to split between five students. Unlike regular organic and synthesis labs where multiple people can be doing separate tasks beneath their individual hoods, our lab’s main work involves using the laser for extended periods of time. Thus, if we aren’t on the “scope” then we are either working up data from scans or wading through the vast sea of research papers.

But enough of that boring stuff…As stated in my initial blog post, silica shells act as a uniform spacer from the surface of the silver nanoparticles to ensure an equal distance between them and a layer of fluorophores. We chose Rhodamine B (RB) dye as a fluorophore because it has an excitation wavelength close to the laser light used in these experiments and because it has previously been utilized in DSSCs. RB dye molecules are spin-coated onto glass coverslips containing silica-coated silver colloid solutions and then we analyze the slide with a confocal microscope. A green laser is used to hit the slide, resulting in fluorescent particles, a photon detector and scanning acquisition program then produces an image, in which particles show up as bright spots. LSPR images are taken of each sample in order to correlate nanoparticle position with fluorescent intensity. The ratio of fluorescence of dye molecules on each nanoparticle is compared to the background to measure enhancement (aka what would make solar cells more effective). The last few sentences probably made zero sense to someone who has yet to take PChem, like me, but here’s the Wikipedia lowdown…The collective and coherent oscillation of electrons caused by light stimulation is called a plasmon and the frequency at which this occurs is deemed the LSPR. LSPR of an excited plasmon can be measured as extinction spectra representing combined absorbance and scattering properties of the particle. The reason that we use the noble metal Ag is because it exhibits extinction maxima in the solar spectrum range. When we hit a silica-coated nanoparticle (SCNP) with the laser, an electron is excited to a higher quantum state and then relaxes to its ground state by emitting a photon of light. This process describes the concept of fluorescence, which is probably the most important aspect of my research. When we calculate enhancement ratios by comparing fluorescent intensities “on particle” to “off particle/background,” we are ultimately measuring how much more sunlight will be absorbed by a DSSC. Here is an example of the images I am describing:


From previous students’ work on this project over the past few years, we have generally affirmed the literature standard that dye-nanoparticle separation ~10nm (thin silica shell) results in the greatest fluorescent enhancement and that thick silica shells display much less enhancement. We have also confirmed that analyzing bare nanoparticles causes quenching to occur, which means the fluorophores actually absorb less light. Still, more research must be done on medium and thin SCNP in order to maximize these enhancement ratios and be able to draw definitive conclusions. Unfortunately, however, it turns out that various people in our group were using different methods to prepare the glass coverslips. One student soaked the slides in sulfuric acid for 24 hours before attaching the colloid solution and spin-coating RB, while another simply went straight to putting the nanoparticles on the slides “untreated.” While this may not seem like a huge deal, it is imperative that the coverslips used in fluorescence microscopy procedures be extremely clean. Although they may appear clean when a new box is initially opened, they may have grease or dust particles on them that could interfere with proper data acquisition. Fluorescent enhancement ratios are useless if the dye on the glass slides form an inhomogeneous background for comparison. As can be seen below from top to bottom, it turns out that acid treated slides produce the most homogeneous background, followed by base-bathed ones, and then untreated slides as the worst. The scans that we had taken for thin SCNP used acid treated slides while the medium SCNP were analyzed on untreated glass. Thus, I will be spending the next week or so looking back through old files to determine which scans need to be discarded due to insufficiently homogeneous backgrounds (and also resolving some trust issues caused by George R.R. Martin).

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