Improved Charge Transport across Bovine Serum Albumin–Au Nanoclusters’ Hybrid Molecular Junction

Proteins, a highly complex substance, have been an essential element in living organisms, and various applications are envisioned due to their biocompatible nature. Apart from proteins’ biological functions, contemporary research mainly focuses on their evolving potential associated with nanoscale electronics. Here, we report one chemical doping process in model protein molecules (BSA) to modulate their electrical conductivity by incorporating metal (gold) nanoclusters on the surface or within them. The as-synthesized Au NCs incorporated inside the BSA (Au 1 to Au 6) were optically well characterized with UV–vis, time-resolved photoluminescence (TRPL), X-ray photon spectroscopy, and high-resolution transmission electron microscopy techniques. The PL quantum yield for Au 1 is 6.8%, whereas that for Au 6 is 0.03%. In addition, the electrical measurements showed ∼10-fold enhancement of conductivity in Au 6 (8.78 × 10–3 S/cm), where maximum loading of Au NCs was predicted inside the protein matrix. We observed a dynamic behavior in the electrical conduction of such protein-nanocluster films, which could have real-time applications in preparing biocompatible electronic devices.

S2. Sample nomenclature: BSA protein with "BSA" represents unmodified BSA protein, whereas "Control" represents BSA protein after adding gold salt, without letting them react to get Au NCs. Different amounts of Au on each BSA molecule (i.e., six different loading morphology) were carried out by altering their effective concentrations during preparation, shown in Table S1. We have used Au 1, Au 2, Au 4, and Au 6 for subsequent measurements to obtain their trends in electrical conduction. Similar small fluorescent Au NCs reported in the literature are listed in Table S2.

S3. X-ray photoelectron spectroscopy (XPS) characterization:
The surface elemental analysis of the sample was determined by X-ray photon spectroscopy (XPS) measurements were conducted using Al Kα (1486.6 eV) radiation (PHI VersaProbe III). The measure was carried out with a detection angle of 45 °C, using pass energies at the analyzer for survey spectra as 6.5 eV and detailed spectra as 280 eV, respectively. The x-ray power provided was 24.17 Watt, and the beam diameter (x-ray spot size) was 100 µm. The exact surface spot size of the sample (5 × 5 µm) was selected for the XPS analysis, which took 20 minutes to complete the survey. The samples were neutralized with electrons from a flood gun (current 20 µA) to compensate for the charging effects of the surface. The energy resolution is ≤0.5eV for our well-tuned instrument. We obtained it by measuring the FWHM of the Ag3d Peak, with data collected by using the least pass energy.
For calibrating the binding energy scale, Au and Cu peaks are used at 83.96eV and 932.62eV, respectively, after the sample is sputter cleaned. PHI VersaProbe III typically calibrates the instrument to within 0.1eV on those peaks. Therefore, we get the calibration peak for Au at (d) S2p shows two peaks attributing to sulfur gold (Au-S) (red curve) and oxidized sulfur (SO2)(blue curve) 14

S4. Electrical Characterization of protein films:
The globular BSA protein has been studied widely by various researchers for numerous years, using varied techniques to illustrate multiple biophysical and biochemical systems. The studies indicate a prolate ellipsoidal shape for defatted BSA molecules having a molecular weight of 66700 ±400 Da. The primary structure of BSA protein comprises 585 amino acid residues, while its secondary structure contains 67% of alpha-helix and 17 disulfide bridges that validate its noteworthy stability. To measure the electrical data from BSA protein film, the Keithley Source-Measurement Unit (2636B SYSTEM Source Meter) instrument was used to measure the electrical characteristics between the two electrodes. One of the two electrodes was the cleaned FTO glass substrate.
In contrast, the second electrode was attached to the protein film with a hanging drop of Eutectic Indium Gallium (EGaIn) to avoid damage to the film, which is only a thickness of about a few μm. KickStart 1.98 software was used to take readings of I-V characteristics for each particular protein concentration in films. The current was recorded for +1V to -1V and -1V to +1V for each set of I-V data. The current had to be converted into current per unit area per unit thickness (normalized current density) of the protein films to normalize the effect of contact area and the thickness variation of the protein films. We have estimated the contact area of the EGaIn droplet from optical images, and the thickness of each film prepared with various protein concentrations was obtained from optical Profilometer measurements. ImageJ software measured the contact area using ImageJ software, from where the radius and contact angle were measured, and the contact area was calculated. One of the examples is explained in detail; other contact areas were also measured in the same way ( Figure S2).

S5. Electrical conductance data analysis method:
The current density is the current divided by the contact area of the material.   The formula for log-normal fit where y0 is y at maximum, xc is center, w is the full-width half maxima (Log standard deviation), and A is the area of the fitted curve.
Conductivity for the highest 12.5 mg/ml was obtained at 1.64 × 10 -5 S/cm, whereas, for 6.25 mg/ml, it showed a decrease as it was recorded as 6.37 × 10 -5 S/cm and 6.55 × 10 -5 S/cm for 2.5 mg/ml of BSA protein concentrations.
In the graphs reported in the main text (Figure 4), the vertical axis represents Conductivity (S/cm), and the horizontal axis illustrates the Au cluster dopant concentrations variance.
There is a reduction in the Conductivity for films prepared with higher protein concentration following larger film thicknesses with more Au clusters in the transport path, which may constrict the charge migration through the thin films.
In our experiments, we have utilized five films for each concentration, and we have prepared around 20-30 different junctions for each film by approaching and retracting the upper electrode.

S7. Temperature-dependent PL emission spectrum of different BSA-Au samples
Temperature-dependent PL characteristics of Au 2 to Au 6 samples were shown in Figure   S3, where the temperature varied from 10 0 C to 60 0 C. In all the cases, better PL intensity was observed in lower temperatures.

S9. Summary of Electrical conductivity data analysis for films prepared with various concentrations:
The total junctions measured, the peak of log-normal fitting obtained along with Conductivity which has maximum occurrences, the width of the fitting curve, and the ratio of the most occurring junction to total junction are given in the table below with the individual graph for each concentration.