The previous four sections have provided a cross-sectional overview of the state of the art in nanobiosensors, with particular attention paid to how well each technology meets the broad application requirements as outlined in Sect. 2
. Before closing this review we will attempt to condense as much of that information as possible down into a few short summary statements.
As was originally mentioned in section two and referred to throughout this review, the primary advantage of nanoscopic biosensors in not necessarily that they exhibit higher internal sensitivity to bulk measures, such as refractive index changes, but rather that the surface area or volume which is probed is much smaller. Given that most of the applications described above are concerned with pushing the limits of how few molecules can be detected in a given volume of solution, nanobiosensors are inherently useful so long as the entire platform (inclusive of the biosensor and associated micro-/nanofluidics) are designed in such a way that the entire sample volume can be interrogated by the sensor. With this in mind, to our knowledge mechanically resonant devices have demonstrated the lowest limits of detection, so long as the measurements can be made in an environment where viscous damping is minimized (usually vacuum but also potentially in air). In liquid environments, where viscous damping is high, the 1D nanostructure electrical detection technologies have, to date, best demonstrated the ability to push the limits of sensitivity. Since most biomolecular detection strategies involve the “chaining” together of a series of molecules (namely a linker, probe and finally target) to perform the measurement, the spatial limitations on the charge field disturbance in moderate ionic strength solutions can in some cases prove to be a significant limitation. As mentioned above, reducing the run buffer ionic strength can help to increase the double layer thickness, but can also serve to impede binding due to enhanced electrostatic repulsion. Optical devices have a similar spatial probing limitation, governed by the thickness of the evanescent field. This tends to be somewhat less restrictive since the evanescent field can be as thick as a few hundred nanometers (Saleh and Teich 1991
). The tradeoff is that the thicker the evanescent field, the more the exposed optical energy is diluted and the greater the probed volume. As such a greater amount of bound mass is required to produce a measurable change in the transduction signal. For systems like the zero-dimensional LSPR nanoparticles shown in , the total surface area is so small that this is more than compensated for.
Surface plasmon resonance imaging techniques as described in Sect. 3.3.5
almost certainly represent the most well developed and cheapest technique for performing label free, highly multiplexed, biosensing. The simplest implementations, however, lack the LOD strength of the nanosensor technologies. As such platforms which incorporate the LOD advantages of the LSPR techniques while maintaining this level of multiplexing are likely to be very successful. The advanced optical devices such as the photonic structures described in Sect. 3.1
and Sect 3.2
have the advantage of being able to operate in a planar format (meaning the excitation source, device, fluidics, and detector can all be in the same plane) make such devices useful for more chip-based devices. To date, however, the LOD and measurement parallelity of these devices has not been as well developed as some other techniques. An advantage of such devices is that the fabrication is not particularly difficult, typically requiring only single layer lithography. One-dimensional nanostructure arrays are particularly promising given the demonstrated sensitivity (subject to the limitations described above) and the inherent ease of using an electrical measurement scheme. The challenges in terms of assembly of nanowires into a device structure are somewhat of a bottle neck at present, however, as the semiconductor industry continues to push the limits of lithography, direct patterning will become more and more feasible (beyond what has already been demonstrated).
Another major challenge to the further development of particle nanosensor technology is one that is not often addressed in the literature, namely the ability to target and confine chemical functionalization to the sensor site itself. For example, sensors which probe only a small area but require functionalization of a larger one (say due to the use of traditional spotting techniques) do not really improve the LOD as binding occurs everywhere and not preferentially at the sensor site. In cases where the background surface chemistry is much different than the sensor itself (say a gold nanoparticle on a silica surface) specific functionalization can be relatively easy. It is more difficult for devices where the surface of the sensor itself is not much different than the background surface. An example of such a case would be silicon on insulator optical devices (SOI) where it is difficult to find chemistries which will work on silicon but not the underside oxide layer.