AVS2013 Session BI+NL+NS+SS-ThM: Bio/Nano Interfaces

Nanoparticles (NPs) provide a well-defined template for preparing supported lipid membranes with controlled curvature. We have coated supported lipid bilayers and hybrid membranes on silica and gold nanoparticles. The localized surface plasmon resonance (LSPR) of gold NPs can be used to monitor the assembly of lipid layers on NPs and to monitor protein lipid interactions. The gold LSPR is very sensitive to the immediate surroundings of the nanoparticle surface and therefore provides a method to monitor the coating of lipids and subsequent conversion of a supported bilayer to a hybrid membrane after the addition of hydrophobic alkanethiols. We demonstrate that both long chain (decanethiol) and short chain (propanethiol) anchors are able to form hybrid membranes and that these membranes allow for LSPR based detection of protein binding events at the membrane surface.

While many materials have been used as membrane supports, there are unmet needs in the development of membrane mimics and it remains challenging to monitor the coating process and to control the curvature of a membrane. Recent work has demonstrated that quantum dots, silica NPs, and gold NPs can be used as templates for membranes providing an opportunity to control curvature. Here, we have exploited the local refractive index sensitivity of the gold LSPR to observe the process of lipid-coating, structural rearrangement of supported membranes into hybrid membranes, and finally the binding of protein. The introduction of phosphatidylcholine (PC) to the gold NPs results in a rapid binding evidenced by a change in the wavelength of the LSPR, however, the interaction with gold is weak and the gold is not completely covered by the lipid. By adding a hydrophobic alkanethiol anchoring group, the lipids bind closer to the gold NP surface resulting in increased stability. This stability is achieved at different concentrations for short and long hydrophobic chains. When propanethiol was used it was possible to destabilize and remove the lipid coating by adding the hydrophilic thiol, beta-mercaptoethanol. This loss of membrane is observed through changes to the LSPR and increased permeability of the membrane to ions.

Ultrasensitive biosensing is one of the main driving forces behind the dynamic research field of plasmonics. I will show that the sensitivity of single metal nanoparticle plasmon spectroscopy can be greatly enhanced by enzymatic amplification of the refractive index footprint of individual protein molecules, so called plasmon-enhanced ELISA. The technique, which is based on generation of an optically dense precipitate catalyzed by horseradish peroxidase at the metal surface, allowed for colorimetric analysis of ultralow molecular surface coverages with a limit of detection approaching the single molecule limit. In addition I will show that by combining large arrays of well-separated gold nanoparticles fabricated by electron beam lithography (EBL) with hyper spectral imaging, spectral responses of up to 700 LSPR particles can be simultaneously studied. This allows us to obtain enough statistical significant number of spectra to further study the inhomogeneous broadening of the sensing properties of individual particles. This includes how variation in electric field enhancement over the surface of a single particle and variation in size and morphology of the enzymatic precipitate could affect the uncertainty in determining the number of enzyme molecules per particle. By combining the electromagnetic simulations with the measurements we could conclude that main sources of uncertainty come from variations in sensitivity across the surface of individual particles and between different particles. There is also a considerable uncertainty in the actual precipitate morphology produced by individual enzyme molecules. I will also discuss the possible improvement that can be done to achieve digital responses from the enzymatic amplified single particle sensing.

The detailed analysis of biospecific interactions is of crucial importance in biomedicine, biotechnology, and pharmacology.Important applications range from medical diagnosis and drug development to the screening of the human genome and proteome. In recent years, array concepts have become very popular and powerful tools to allow for highly parallel, rapid identification of binding events. These arrays contain a multitude of different probe molecules immobilized at specific locations of an underlying substrate.

Interaction analysis is usually facilitated by labelling the potential binding partners with additional markers. Today, many of such techniques are well-established yielding considerably low detection limits and high lateral resolution. Yet, they suffer from the fact that labelling procedures are usually costly and time-consuming, that the labelling efficiency needs to be properly controlled for quantitative analysis, and that the marker itself can affect the original functionality of the molecules being studied. Moreover, the detection of low-affinity binding events is often hampered as additional washing steps and (bio)chemical reactions are required in-between the interaction and detection processes. To overcome those obstacles, strong efforts have been made to establish label-free detection schemes for interaction analysis. However, marker-free, sensitive readout of high-density arrays is still a technological challenge.

Graphene, a one-atom thick sheet of sp² carbon, offers many intriguing possibilities in the field of molecular sensing. Its unique combination of large areas with nanometer thickness and high electrical conductivity could enable small scale device sensitivity with large scale production methods. A major benefit of using graphene is the large toolbox of well-established chemistries for incorporating chemical functionalities or specific recognition elements at the device surface. Here, we will discuss our efforts to develop graphene-based biological field-effect transistors (BioFETs), which offer sensitivity comparable to sensors made with other nanoscale materials (carbon nanotubes, nanowires), but with greatly simplified production methods common in the semiconductor industry. Devices utilizing both graphene and graphene oxide will be covered, and surface spectroscopic studies of the material modification will be discussed. Successful results for the detection of specific DNA hybridization using graphene BioFETs will also be presented. We will further discuss our efforts to use graphene as a biofunctionalized interface for a number of materials, from polymers to dielectrics to semiconductors, of interest to the biosensing community. Graphene’s ultrathin nature allows its inclusion in more traditional sensing platforms as a non-intrusive functionalization layer, discreetly lending its chemical flexibility to other, more inert materials without significantly impacting the sensing device.