Biomedical surface science: Foundations to frontiers

Surfaces play a vial role in biology and medicine with most biological reactions occurring at surfaces and interfaces. The foundations, evolution, and impact of biomedical surface science are discussed. In the 19th century, the first observations were made that surfaces control biological reactions. The advancements in surface science instrumentation that have occurred in the past quarter of a century have significantly increased our ability to characterize the surface composition and molecular structure of biomaterials. Similar advancements have occurred in material science and molecular biology. The combination of these advances have allowed the development of the biological model for surface science, where the ultimate goal is to gain a detailed understanding of how the surface properties of a material control the biological reactivity of a cell interacting with that surface. Numerous examples show that the surface properties of a material are directly related to in vitro biological performance such as protein adsorption and cell growth. The challenge is to fully develop the biological model for surface science in the highly complex and interactive in vivo biological environment. Examples of state-of-the-art biomedical surface science studies on surface chemical state imaging, molecular recognition surfaces, adsorbed protein films, and hydrated surfaces are presented. Future directions and opportunities for surface scientists working in biomedical research include exploiting biological knowledge, biomimetics, precision immobilization, self-assembly, nanofabrication, smart surfaces, and control of non-specific reactions.     

Hydrolysis of microporous polyamide-6 membranes as substrate for in situ synthesis of oligonucleotides

This article provides a novel method of preparing substrate for in situ synthesis of oligonucleotide by hydrolyzing microporous polyamide-6 membranes in a 0.01 mol/l/NaOH/(H₂O–CH₃OH) mixture medium with refluxing about 36 h. Attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) demonstrated the emergence of amines (NH₂) on the surface. Optimum hydrolyzing conditions were determined through the ultra-violet (UV) spectra. A pH value of 12 and a hydrolysis time of 36 h are the preferred conditions for the modification. The treated membrane can be applied to in situ synthesis of oligonucleotide and, for example, the oligonucleotide probes of 5^′-AAC CAC CAA ACA CAC-3^′ were successfully synthesized on the hydrolyzed membrane. The single step coupling efficiency determined by ultraviolet (UV) spectra is above 98%.     

Computation with DNA on surfaces

DNA computation has the potential to tackle computationally difficult problems that have real-world implications. The parallel search capabilities of DNA make it a valuable tool to approach problems that have a large number of possible solutions, for which conventional computers have limited potential. Surface science can play a significant role in harnessing the parallel nature of DNA for computation. This article briefly reviews conventional computing architecture, discusses DNA computation, and describes the role of surface science in DNA computation.     

The surface science of semiconductor processing: gate oxides in the ever-shrinking transistor

Due to the extreme dimensional scaling required by Moore's law, Si device technology is increasingly subject to the limitations imposed by the intrinsic physics and chemistry of surfaces and interfaces. In this review we outline ways in which fundamental surface science has contributed an understanding to the microelectronics community and discuss areas where surface science may impact future development. We focus on the example of silicon dioxide (SiO₂) on silicon, since this interface lies at the heart of modern transistor technology and has therefore received a great deal of attention in recent years. We highlight a number of experimental and theoretical approaches that have elucidated the fundamental phenomena associated with the formation and evolution of this critical technological interface, revealing the remarkable interdependence of science and technology that now characterizes this rapidly evolving industry.     

Immobilization of protein molecules on step-controlled sapphire surfaces

We have studied effects of surface morphology on immobilization of protein molecules using step-controlled sapphire surfaces. Preferential adsorption of avidin molecules on the step edges was observed on the single-stepped sapphire surface. A randomly-stepped sapphire surface was found to be suitable for high-density immobilization of protein molecules. These results indicate atomic scale structures of the substrate surface influence the adsorption efficiency of the proteins. By using an atomic force microscopy (AFM) equipped with a biotin-modified cantilever, we have confirmed that the immobilized avidin molecules on the substrates keep their biological activity. This means that the ligand-receptor interaction can be detected using the phase image mode of a standard AFM.     

Nanoscale depth-resolved cathodoluminescence spectroscopy of ZnO surfaces and metal interfaces

The electronic properties of ZnO surfaces and interfaces has until recently been relatively unexplored. We have used a complement of ultrahigh vacuum scanning electron microscope (SEM)-based, depth-resolved cathodoluminescence spectroscopy (DRCLS), temperature-dependent charge transport, trap spectroscopy, and surface science techniques to probe the electronic and chemical properties of clean surfaces and interfaces on a nanometer scale. DRCLS reveals remarkable nanoscale correlations of native point defect distributions with surface and sub-surface defects calibrated with capacitance trap spectroscopies, atomic force microscopy, and Kelvin probe force microscopy. The measurement of these near-surface states associated with native point defects in the ZnO bulk and those induced by interface chemical bonding is a powerful extension of cathodoluminescence spectroscopy that provides a guide to understanding and controlling ZnO electronic contacts.     

The surface science of enzymes

One of the largest challenges to science in the coming years is to find the relation between enzyme structure and function. Can we predict which reactions an enzyme catalyzes from knowledge of its structure—or from its amino acid sequence? Can we use that knowledge to modify enzyme function? To solve these problems we must understand in some detail how enzymes interact with reactants from its surroundings. These interactions take place at the surface of the enzyme and the question of enzyme function can be viewed as the surface science of enzymes. In this article we discuss how to describe catalysis by enzymes, and in particular the analogies between enzyme catalyzed reactions and surface catalyzed reactions. We do this by discussing two concrete examples of reactions catalyzed both in nature (by enzymes) and in industrial reactors (by inorganic materials), and show that although analogies exist and the two kinds of catalyst can be described by similar tools, nature and human effort have come up with different solutions. This on the other hand implies that new and improved catalysts may be made by learning from nature.     

Biological functionality of active enzyme structures immobilized on various solid surfaces

We have investigated biological functionality of immobilized enzyme structures according to the immobilizing routes and the surface properties. Horse radish peroxidase (HRP) was immobilized on various solid surfaces such as gold, SiO₂, sapphire and anodized aluminum oxide (AAO) membrane via non-specific adsorption, avidin-mediated and biotin/avidin-mediated layer-by-layer (LBL) assembly. The catalytic activity as a measure of biological functionality, of the biotin-HRP immobilized by avidin-mediated LBL assembly was found to be better than that of the directly adsorbed HRP on the surfaces of gold, SiO₂, sapphire and AAO due to the easy accessibility of reactants to active sites as well as the retention of three dimensional native structure of enzyme for bioactive functionality. In addition, the catalytic activity of the biotin-HRP in LBL-assembled avidin/biotin-HRP on AAO membrane was found to be highly better than that on other substrates due to the increasing amount of immobilized HRP which can be attributed to the high surface area of the substrate. SEM images show that the functional avidin/biotin-HRP enzyme structures were successfully realized by a sequential process of non-specific adsorption and LBL assembly via biotin–avidin interaction.     

The IR spectra of Mg5C(CO) complexes on the (0 0 1) surfaces of polycrystalline and single crystal MgO

The FTIR spectroscopy of carbon monoxide adsorbed on polycrystalline MgO smoke has been investigated as a function of the CO equilibrium pressure at constant temperature (60 K) (optical isotherm) and of the temperature (in the 300–60 K range) at constant CO pressure (optical isobar). In both cases the spectra fully reproduce those of CO adsorbed on the (0 0 1) surface of UHV cleaved single crystals [Heidberg et al., Surf. Sci. 331–333 (1995) 1467]. This result, never attained in previous investigations on dispersed MgO, contribute to bridging the gap which is commonly supposed to exist between surface science and the study of “real” (defective) systems. Depending on the surface coverage θ the main spectral features due to the CO/MgO smoke interaction are a single band shifting from 2157.5 (at θ→0) to 2150.2 cm⁻¹ (at θ=1/4) or a triplet, at 2151.5, 2137.2 and 2132.4 cm⁻¹ (at θ>1/4). These manifestations are due to the ν(CO) modes of Mg_5C²⁺· · · CO adducts formed on the (0 0 1) terminations of the cubic MgO smoke microcrystals. The formation of the CO monolayer is occurring in two different phases: (i) a first phase with CO oscillators perpendicularly oriented to the surface (2157–2150 cm⁻¹) and (ii) a second phase constituted by an array of coexisting perpendicular and tilted species (triplet at 2151.5, 2137.2 and 2132.4 cm⁻¹). A much weaker feature at 2167.5–2164 cm⁻¹ is assigned to Mg_4C²⁺· · · CO adducts at the edges of the microcrystals. The heat of adsorption of the perpendicular Mg_5C²⁺· · · CO complex in the first phase has been estimated from the optical isobar and results to be 11 kJ mol⁻¹.