Morphology Effect on Charge Transport in Doped Bovine Serum

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Morphology Effect on Charge Transport in Doped Bovine Serum Albumin Self-Assembled Monolayers Edith Beilis, Yonatan Horowitz, Alon Givon, Gabor A. Somorjai, Hagai Cohen, and Shachar Richter J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b01355 • Publication Date (Web): 13 Apr 2017 Downloaded from http://pubs.acs.org on April 17, 2017

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The Journal of Physical Chemistry

Morphology Effect on Charge Transport in Doped Bovine Serum Albumin Self-Assembled Monolayers Edith Beilis,1,2 Yonatan Horowitz,3,4 Alon Givon,5 Gabor A. Somorjai,

3,4

Hagai Cohen,6* Sha-

char Richter*1, 5 1

Tel Aviv University Center for Nanoscience and Nanotechnology, Tel Aviv University, Tel-

Aviv, Israel 2

School of Chemistry, Raymond and Beverly Sackler Faculty of Exact Sciences, Tel Aviv Uni-

versity, Tel-Aviv, Israel 3

Materials Sciences Division, Lawrence Berkeley National Laboratory, 1 Cyclotron Road,

Berkeley, California 94720, United States 4

Department of Chemistry, University of California, Berkeley, California 94720, United States

5

Department of Materials Science and Engineering, Tel Aviv University, Tel-Aviv, Israel

6

Department of Chemical Research Support, The Weizmann Institute of Science, Rehovot

76100, Israel

ACS Paragon Plus Environment

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ABSTRACT: Direct exploration of the mutually interfering morphological and charge transport characteristics in self-assembled monolayers (SAMs) is reported. These strongly coupled properties are addressed by means of surface spectroscopy techniques, combined such as to consistently account at high sensitivity for a broad range of surface properties without any use of a top contact. Applied to doped bovine serum albumin (BSA) SAMs, we show how the BSA conformation, its dehydration and monolayer assembly are all correlated. Moreover, the electrical properties, transport and charge trapping, are highly affected by the SAM compositional and structural state, with a specific roll of water molecules. Our results reveal and further demonstrate a useful approach to the complex challenges presented by (bio) molecular electronics.


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INTRODUCTION Bioelectronics focuses on the realization of electronic junctions in which biomolecules are utilized to transfer or generate electric signals1,2,3. In this respect, proteins offer significant advantages over other bio-polymers since they exhibit specific interfacial recognition properties4. Moreover, since plasma proteins such as Serum Albumins and Fibrinogen are extensively used in blood-contacting implant devices, biosensors, and drug delivery studies5, detailed understanding of their adsorption processes and resulting layer's morphological and electrical properties is of significant technological importance. In solution, the electron transfer processes involving protein molecules are often tuned by the distance between donor (D) and acceptor (A) groups6 which are localized within the biological macromolecules, the intra-molecular organization7 and the medium in the vicinity of the protein's redox sites8,9. Nevertheless, this is not the case when one explores the electron transport (ETp) across a single protein molecule or self-assembled protein monolayers (SAMs) in a dry environment10. In this case, ETp is governed by the electronic coupling between the protein molecules and the conducting electrodes, as well as the intramolecular properties such as the electron conduction path via distinct secondary structures, mostly the α-helices11,12. Importantly, once the proteins are taken out of their natural aqueous environment and deposited on solid surfaces, their morphological properties, such as their conformation, orientation, and residual tightly bound water molecules, may change dramatically upon adsorption13,14. This, in turn, is expected to affect significantly the charge transport properties of their resultant SAMs. Therefore, the exploration of this effect is of great importance. Assessment of the relations between the protein-based SAM morphology and its charge transport characteristics is challenging since it requires applications of methods that can detect nm-sized structural changes and couple them to electrical


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properties. The challenge is enhanced while exploring the common molecular junction at which an upper metallic layer is introduced on top of the adsorbed SAM thus inducing additional morphological changes at the organic layer, combined with unknowns about the contact itself and its interface to the molecules. In a previous work15, we demonstrated how changes, during adsorption, in the internal structure of the protein affect the resulting SAM morphology. We further showed how doping the protein with small organic molecules (prior to adsorption) enabled partial control over these properties. We found that doping affects the protein conformation (e.g. secondary structure and orientation) and, thus, affects the resultant surface coverage and protein's water content. In this work, we take this issue one step further. We reveal the direct correlation between morphological properties of protein-based SAMs and their charge transport characteristics. Both the morphology and transport features were varied by means of doping, which provided useful information on the two related aspects. Complementary surface sensitive spectroscopic techniques were applied to extract data without any use of a top contact, neither for detection nor as a power supply. Sample architecture consisted of a non-encapsulated surface, available for in-situ structural and compositional

studies,

which

presents

marked

advantages

over

electrical

measurements with a top contact. The spectroscopic techniques used here include Chemically Resolved Electrical Measurements (CREM)

16

, coupled to X-ray photoelectron spectroscopy

(XPS), polarization modulation infrared reflectance absorption spectroscopy (PMIRRAS) and sum frequency generation (SFG) vibrational spectroscopy17,18. The protein chosen in this study is Bovine Serum Albumin (BSA, Figure 1). It exhibits natural ability to bind smaller, mainly hydrophobic, molecules (dopants) and a proven capacity to form SAMs on solid metal surfaces. Unlike certain types of metalloproteins that are involved in biological ET processes (such as Az-


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urin and Cytochrome-C), BSA has no redox activity center or porphyrin cofactor (a.k.a heme). For this purpose, BSA-doped with redox molecules (tetraphenyl-21H,23H-porphine (TPP ) and its Cu and Fe metallo- derivatives (TPPCu; TPPFe respectively), Figure 1) SAMs were formed in a procedure described elsewhere15.

Figure 1: a) schematic presentation of the molecular structure of a.1) TPP a.2) TPP-Fe or TPPCu b) Docking simulation15 of TPP doped BSA: α helices are marked in red, negatively charged residues are indicated in purple, positively charged residues are indicated in blue.

METHODS Materials. Purified Bovine serum albumin (BSA, fraction V, lyophilized powder) was purchased from Sigma Aldrich and used as received. Sodium Phosphate Buffer was prepared using sodium salts: sodium phosphate monobasic and sodium phosphate dibasic to give pH 7.2-7.4 (200 mM) at room temperature. The buffer stock solution was filtered and then diluted with deionized water (resistivity 18.2 Ωcm) to 10 mM. Stock protein solution in diluted buffer was pre-


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pared immediately before use. 5,10,15,20-Tetraphenyl-21H,23H-porphine copper(II) (TPP-Cu), 5,10,15,20-Tetraphenyl-21H,23H-porphine iron(III) chloride (TPP- FeCl), and 5,10,15,20Tetraphenyl-21H,23H-porphine (TPP), purchased from Sigma Aldrich, were used as received. Stock solutions of the three compounds dissolved in Dimethyl Sulfoxide (DMSO) were prepared immediately before use. TPPFeCl solubility in DMSO/Buffer mixture was the best resulting in a homogenous solution. On the other hand, TPP and TPPCu formed semi-homogunous suspensions with some aggregation observed. To improve the homogeneity, all the solutions were sonicated prior to incubation with BSA. Samples Preparation. The sample preparation procedure done here is based on a protocol described in detail elsewhere15. Briefly, SAMs of un-doped-BSA and the three differently doped BSA complexes were prepared by immersing annealed hydrophilic gold samples for 20 h in freshly prepared buffered complex solutions, after which the samples were removed from solution rinsed with DI water and dried with nitrogen flow. PMIRRAS Measurements. PM-IRRAS spectra were recorded using Bruker optics PMA50 external module in conjunction with a Vertex 70 FT-IR spectrometer. A liquid nitrogen-cooled MCT detector was used in all experiments. The incident beam angle used in all experiments was 85°. The wavelength setting on a Hinds PEM90 was fixed at 1600cm-1 with a half-wave retardation

of

0.5.

The

samples

were

scanned

for

10-20

min

with

4 cm-1 resolution and an aperture setting of 1-1.5 mm depending on the signal amplitude. The resulting absorption spectra were recorded between 800 and 4000 cm-1. Conversion of the resulting interferograms to raw PMIRRAS spectra was performed using Visual-Basic Script included in Bruker OPUS® Spectroscopy V.5.5 software. Analysis of the raw PMIRRAS


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spectra, includes removal of background, y-scale normalization (using gold substrate spectra) and removal of the LIA gain factor (5mV-10mV). X-ray Photoelectron Spectroscopy and Chemically Resolved Electrical Measurements. The XPS measurements were performed using both Kratos AXIS-HS and Kratos Ultra-DLD spectrometers with monochromatic Al Kα source (1486.6eV) at low power, 75 W, and base pressure

Morphology Effect on Charge Transport in Doped Bovine Serum

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