Relevant Aspects of Surface Physical Chemistry

1.1 Introduction

Advances in biosciences, the discovery of new drugs, the development of novel clinical diagnostic methods, and improvements in food safety rely heavily on modern analytical techniques. These technologies provide scientists with the ability not only to detect analytes, biomarkers, or specific pathogens, but also to quantify biochemical and biophysical interactions among biomolecules and cells. The sensitive monitoring of biomarkers allows the early detection of disease before its progression, opening the possibility of treatments with the highest probability of success.

Generally, the methods used in routine biochemical analyses are based on immunoassays and require the use of labeled antibodies as reagents. A further step, such as an enzymatic reaction, is usually necessary to produce a physical signal that can be read. However, affinity biosensors are able to quantify the analyte of interest without using additional reagents. They rely on a transducer element that can directly translate the concentration of analyte in its immediate vicinity into a physical signal. The transducer surface must be equipped with appropriate biorecognition elements that can bind the analyte. The advantages of label-free measurements include rapid analysis times, real-time monitoring of concentrations, the possibility of measuring the binding kinetics, and ease of automation and miniaturization. Automation and miniaturization are critical, as they offer the possibility of measuring a large number of analytes in parallel, with high throughput. Such detection methods rely on the conjugation of bioreceptors. Their continuous development depends both on advances in the design of physical sensors and the implementation of novel sensing principles, and on the precise control of the interactions between these artificial devices and the biological fluids/environments in which they are meant to operate.

1.1.1 Materials and Biological Systems – The Biomaterials Interface

An effective affinity biosensor must be able to concentrate the analyte from the solution on its surface without eliciting non-specific interactions with other components present in the medium. While the transducer is able to generate a readable signal due to the presence of the analyte, it generally cannot discriminate between changes arising from the analyte concentration and those arising from the concentrations of interfering substances. The biological fluids of interest for biosensing are highly complex matrices, such as blood plasma and serum, which will be discussed in depth in later chapters. They contain an enormous and varied range of potential interferents.

The interactions of biosensors with biological fluids are critical for determining their performance; nonetheless, many applications are affected by such interactions. In fact, the whole category of biomaterials can be defined as materials (other than food and drugs) that are engineered to be used, either independently or as part of a larger system, in human or veterinary medicine for diagnostic or therapeutic procedures involving continuous contact with biological fluids. It is clear from this definition that affinity biosensor surfaces fall within this category as they can perform their function only while they are in contact with the biological matrix in which the analyte is present. Moreover, bioimplants and therapeutic devices which come into contact with biological fluids are subject to the same phenomena. Even outside the field of medical practice, biological and biophysical research is constantly faced with events that occur when artificial and biological systems come into contact.

For a biomaterial to support the interactions at the interface required for each application, the surface must be functionalized with appropriate bioreceptors. These molecules can bind a specific target in solution or, more generally, perform or promote a specific function. Examples include, but are not limited to, antibodies, enzymes, aptamers, oligonucleotides, receptor proteins, molecularly imprinted polymers (MIPs), peptide motifs, and even whole cells. However, they need to be immobilized in such a way as to ensure a sufficient quantity, homogeneous distribution, appropriate spatial orientation, and preservation of the bioreceptor structure and conformation. Particular applications may impose additional requirements, such as site specificity on the bioreceptor molecule or spatial selectivity to create patterns on the surface.

However, upon contact of artificial materials with complex biological media, components of the fluid may be spontaneously deposited on the surface. This non-specific adsorption, called fouling, can dominate the biomaterial–biological fluid interaction, ultimately impairing the performance of a device.

1.1.2 The Problem of Fouling

The biological components most frequently responsible for fouling on biomaterials are proteins. Their complex structures contain regions with varying hydrophilicity and hydrophobicity, and also charges that can mediate interactions with surfaces via multiple mechanisms. Chapter 2 will provide an in-depth discussion of proteins and the origins of fouling. The deposition of a layer of protein on a biomaterial surface immediately and irreversibly alters its properties. This can affect biosensors in two ways. First, the presence of the fouling layer can occlude the immobilized biorecognition elements, preventing the specific binding of the analyte from taking place. Second, label-free affinity biosensors function by directly detecting the analyte on the basis of its intrinsic properties (e.g., mass, dielectric permittivity, and electrochemical redox properties), so the properties involved in fouling can themselves give rise to a detection signal. Since the attachment is non-specific and does not depend on the presence of the biorecognition elements, a signal produced by fouling can be orders of magnitude stronger than the signal related to the specific analyte. This source of interference can render the sensor inoperable, especially for media containing high concentrations of protein in solution. This is why biosensing in these media is considered particularly challenging.

A layer of deposited protein resulting from fouling has critical consequences for implantable and therapeutic devices. The proteins on the surface mediate the subsequent in vivo response, giving cues for an immune response. This leads to foreign body...