Conformational Switching in Nanofibers: A New Bioelectronic Interface for Gas Sensors

Sezer Ozenler,3’* Miige Yucel,b * and Umit Hakan Yildiz3

  • 3 Department of Chemistry, izmir Institute of Technology, Urla,
  • 35430 Izmir, Turkey

b Department of Bioengineering, Izmir Institute of Technology, Urla,

35430 Izmir, Turkey This email address is being protected from spam bots, you need Javascript enabled to view it

Introduction

Switchability is a key mechanism living systems use to respond to an external stimulus in a short period of time. The switching capability relies on a reversible conformational change of the biomacromolecules that enhances the ability of living systems to adapt to the extreme environmental conditions. From primitive single

* Equal contribution by these authors.

Switchable Bioelectronics Edited by Onur Parlak

Copyright © 2020 Jenny Stanford Publishing Pte. Ltd.

ISBN 978-981-4800-89-1 (Hardcover), 978-1-003-05600-3 (eBook) www.jennystanford.com cells to complex organisms, all living systems have the ability to respond to and adapt reversibly to the changes in their surroundings, for instance, heat shock in bacteria,1 change of wettability and/or adhesion skills in gecko and mussels,2,3 and control and regulation of the transport of ions or molecules (cells).4 However, artificial materials and devices show irreversible behavior under an external effect—they have an unchangeable and fixed form and function because of an immobilized biomolecule.5

Mimicking the switchability properties of a living system is an emerging research topic in the field of bioelectronics since the current demand in sensing requires smart biointerfaces. Switchability is often provided by functional molecules6,7 or monolayers (self-assembly monolayers),8 polymers and polymer nanoparticles,9,10 and proteins.11

In this chapter, we present the use of ID polymer nanofibers exhibiting elastic memory that enables rapid switching. This approach emerges as a new alternative for the utilization of smart and switchable biointerfaces. Poly(vinylidene fluoride) (PVDF) nanofibers are selected as a model system for the application of gas sensing. PVDF nanofibers act as a ID elastic material yielding reversible width change upon gas adsorption. This property has been exploited to fabricate thin-layer switchable sensing platforms for the analysis of exhaled breath. Switchability relies on the elastic memory of polymer chains. This chapter begins with a discussion of the theory of polymer elasticity. The basics of conformational change in a polymer chain and energy-related shape resistance are introduced in the theory section. In the next section, the correlation between the effect of the chemical environment, the structure of polymers, and switchability is discussed. Single?molecule force spectroscopy (SMFS) is also introduced to characterize the switchability and shape resistance of polymer chains. Next, recent studies are briefly reviewed to support the link between switchability and chain elasticity. In the last section, switchability and sensing behavior of nanofibers are elaborately explained by our results.

Conformational Change and Energy-Related Shape Resistance of a Single Chain

Theory

Polymers consist of covalently bonded repeating units that cause certain rotational constraints, thereby providing shape-resistant memory or elasticity. The chemical composition of the polymer plays a major role in the chain flexibility, the rotational degree of freedom, and the macroscopic elasticity. The long polymer chains exhibit diverse conformations and configurations, leading to the transformation of the chain position from one to another without breaking the chemical bonds by the simple rotation of the units. However, every molecule is located in space with the lowest possible potential energy and for the conformational change to occur, it is necessary for the energy to exceed a certain value, called the "potential rotation barrier" or the "hindrance potential." When a force exceeding the potential rotation barrier is applied, the polymer chain starts changing conformation to a certain extent, and removing the force induces reorganization of the polymer chain to the initial state.12

As explained earlier polymers exhibit conformations ranging from tight coils to highly extended structures, such as rigid rods and rigid helical chains. The parameters and, to some extent, the methods utilized for evaluating the degree of chain flexibility are usually different for "flexible" and "stiff” chains. For flexible polymers the most common parameter in use is Flory's13-15 characteristic ratio Coo, defined as

where 2>0 is the unperturbed (theta condition) mean- square end-to-end distance, N is the number of main chain bonds of length /, M is the molecular weight of the polymer, and mo is the average mass per main chain bond. Coo is a quantitative measure of the impact of the hindered rotation about the main chain bonds and rather fixed bond angles on 2>. For a freely jointed chain (FJC), which has neither rotational hindrance nor bond angle restrictions, 2> is equal to /V/2.16-19 Thus, the value of is equal to 1 for the FJC and larger Coo values are indicative of greater departures from the freely jointed character, that is, diminished flexibility.

The relative contributions of fixed bond angles and hindered rotation can be elucidated by modification of the FJC model. Eyring20 showed that when N is very large 2> can be calculated as

where в is equal to 180° minus the fixed bond angle. Thus, for a polyethylene backbone cos в = 0.333 and 2> = 2Nl2 with fixed tetrahedral bond angles for a saturated hydrocarbon backbone being expected to double 2>. In light of the above, an alternative chain flexibility parameter, which is commonly used, is the conformation (steric) factor a:

This is obtained from the experimental 2>о value, and the 2> value calculated for the freely rotating chain, that is, 2> f a, provides a measure of the relative increase in the end-to-end distance brought about by hindrances to rotation only. Obviously, Cx and a are related for tetrahedral hydrocarbon backbones by

As Flory has pointed out,21 the use of Coo is to be preferred over the use of a. This is because small changes in bond angles can lead to large changes in a and bond angles are rarely known to within more than a few degrees.

The effect of restricted rotation can be taken into account by introducing an additional term into Eq. 4.2. If the hindering potentials are mutually independent for neighboring bonds and symmetrical,22-24 2> becomes

Here

is the average rotation angle (ф = 0° for trans). The assumptions upon which Eq. 4.5 is based are invalid for many macromolecules. Of great significance in this regard has been the development of the rotational isomeric state (RIS) model.13'14,25'26 These models generally consider only a few discrete conformers of relatively low energies. Interdependence of bond rotations, asymmetric

A segment of a persistent chain

Figure 4.1 A segment of a persistent chain.

rotational potential curves, and finite chain length effects can all be dealt with using modern RIS methods.13,14

The Kratky-Porod wormlike chain (WLC) model27,28 is widely used for describing conformational characteristics of less flexible chains. The polymer is viewed as semiflexible stirring (or worm) of the overall "contour length" L with a continuous curvature. The chain is subdivided into N segments of length AL, which are linked at a supplementary angle The persistence length q (Fig. 4.1) is defined as29

and is thus a measure of the tendency of segments in the polymer chain to "remember” the orientation of adjoining (and other) segments in the chain. WLCs will exhibit conformations ranging from random coils to rigid rods depending on the value of the ratio L/q. Therefore, q provides a measure of chain stiffness. Furthermore, it can be shown29 that at a large L a WLC becomes Gaussian and q is related to the length of the Kuhn statistical segment /' as

Thus, the value of /' is also commonly taken as a measure of chain stiffness. Flory21 has shown that there is also a simple relationship between the persistence length and the characteristic ratio

Here / is the average bond length and oo indicates, as usual, the limiting value for infinite chains. Hence, C00 can also be employed in comparing the relative stiffness of WLCs.15

Shape-resistant memory, or elasticity, of a polymer chain is a consequence of the interplay between intra- and intermolecular forces acting on each repeating unit.

M-FJC and WLC Models

The modified freely jointed chain (M-FJC) model is utilized to identify the extension of a polymer and produce restoring forces of entropy. In this model a macromolecule is considered as a chain of Kuhn length segments [Ik) that are deformable under stress and statistically independent of each other in Scheme 4.1.30

Langevin function describes the extension relevant to the external force:

Scheme 4.1 Schematic drawing of the WLC model.

Here, F is the external force, x is the extension of the polymer (end-to-end distance), Lc is the contour length, n is the number of chains stretched, kB is the Boltzmann constant, T is the temperature, and Ks is the deformability of a segment.

While entropic contribution dominates the elasticity of M-FJC in a low-force region, both its entropy and enthalpy are effective in a high-force region.

WLC is another model that describes the polymer as a homogenous string with fixed bending elasticity. Entropic and enthalpic contributions are combined in the WLC model.31'32

The force and extension of the WLC is shown in the following equation:

Here, /p is the persistence length.

Force Spectroscopy Applications

SMFS is a technique based on atomic force microscopy (AFM) that allows studies at the molecular level. The force signals cross extension curves and provide new knowledge that cannot be obtained in conventional methods.30 Here we give a broad overview of SMFS for characterization of switchability properties of single-polymer nanofibers.

SMFS is a technique that quantifies the interaction between a solid substrate hosting polymer chains physically or chemically adsorbed and the AFM tip. The AFM tip approaches the polymer on the substrate and makes contact with the surface by forming a bridge between polymer and tip. When the AFM tip stretches, the force- versus-extension relation can be deduced to characterize the elasticity of the single chain.30

Single-Chain Elongation

The single-chain elongation of polyfferrocenyldimethy- lsilane) (PFDMS) was studied.33 Figure 4.2a shows that the force curve of the single-chain elongation curve of PFDMS in tetrahydrofuran (THF) contains one force signal in each curve, which shows the single-chain elongation.

The anchor point of the polymer to the tip or surface is stochastic, and the contour length is varied because the polymer molecular weight is polydisperse. The contour length is scaled by normalizing, which includes dividing the PFDMS force curves by relative extension with the same value of forces. Figure 4.2a indicates the linear proportion between the stretching forces and the relative extension.33

According to these force signals all single-polymer chains should superimpose after normalization.34 The PFDMS polymer chain can be stretched and relaxed continuously by maintaining holding the stretching forces to a point below the rupture point (Fig. 4.2b). This indicates an equilibrium situation, and it is reversible process.33 As another step in the study an SMFS experiment of oxidized PFDMS and poly(ferrocenylmethylphenylsilane) (PFMPS) in THF buffer has been done. These two polymers have shown different single-molecule-forces curves. In the same way as the preoxidation step, the normalized force- extension (F-E) curves of both polymers superimposed well. And so, there is no hysteresis between trace and

(a) Several typical force curves of PFDMS in THF buffer

Figure 4.2 (a) Several typical force curves of PFDMS in THF buffer.

One of the force curves is fitted by the M-FJC model curve, shown by the dashed line. Inset: Superposition of the normalized force curves, (b) Successive manipulation of a PFDMS single chain, suggesting that the elongation in the experiment is reversible.

retrace curves for either of them after oxidation. The F- E curves of pre- and postoxidized PFDMS in THF buffer overlap in the low-force regime but branch off in the high- force regime, as shown in Fig. 4.3a.

These two forms of a PFDMS single chain show similar entropic elasticity, but the enthalpic elasticity is larger for oxidized poly(ferrocenyldimethylsilane) (o-PFDMS).

(a) Comparsion of the normalized force-extension curves

Figure 4.3 (a) Comparsion of the normalized force-extension curves

between PFDMS and o-PFDMS in THF buffer, (b) Comparison of the normalized force-extension curves between PFMPS and o-PFMPS in THF buffer.

The PFMPS has a similar tendency in Fig. 4.3b with a steeper form compared to its normal form. The elastic difference between o-PFDMS and oxidized poly(ferrocenyl- methylphenylsilane) (o-PFMPS) can be attributed to the steric effect of the side groups. In oxidized forms Fe+3 should interact with the electron-rich phenyl groups stronger than the methyl groups. Enthalpy elasticity of these polymers is similar when they are in their normal form, but the difference can be accurately seen when they are in their oxidized states.

 
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