Chapter 7 – Complementary Experimental Tools  291

can fluorescently excite fluorophore-​tagged biomolecules and captures their fluorescence

emissions. In other words, this is an ideal technology for developing miniaturized biosensors.

A promising range of nanophotonics biosensor devices use either evanescent field exci­

tation or plasmon excitation or a combination of both. For example, a flow cell can be

microfabricated to engineer channels for flowing through a solution of fluorescently labeled

biomolecules from a sample. The waveguiding properties of the silicon-​based channel can

result in total internal reflection of a laser source at the channel floor and side walls, thus

generating a 3D evanescent excitation field that can generate TIRF in a similar way to that

discussed previously for light microscopy (see Chapter 3). Precoating the channel surfaces

with a layer of metal ~10 nm thick allows surface plasmons to generate, in the same manner

as for conventional SPR devices (see Chapter 3), thus presenting a method to generate kin­

etics of binding data for label-​free non-​fluorescent biomolecules if the channel surfaces are

chemically functionalized with molecules that have high specific binding affinities to key

biomolecules that are to be detected (e.g., specific antibodies). These technologies also can

be applied to live-​cell data.

The advantages of nanophotonics for such biosensing applications include not only mini­

aturization but also improvements in high-​throughput sensing. For example, multiple par­

allel smart flow-​cell channels can be constructed to direct biological samples into different

detection areas. These improve the speed of biosensing by not only parallelizing the detec­

tion but also enabling multiple different biomolecules to be detected, for example, by using

different specific antibodies in each different detection area. This ultimately facilitates the

development of lab-​on-​a-​chip devices (see Chapter 9).

Three-​dimensional printing has emerged recently as a valuable, robust tool. For example,

many components that are used in complex biophysical apparatus, such as those used in

bespoke optical imaging techniques, consist of multiple components of nonstandard sizes

and shapes, often with very intricate interfaces between the separate components. These can

be nontrivial to fashion out conventional materials that are mechanically stable but light,

such as aluminum, using traditional machining workshop tools, in a process of subtractive

manufacturing. However, 3D printing technology has emerged as a cost-​effective tool to gen­

erate such bespoke components, typically reducing the manufacturing time of traditional

machining methods by factor of two or more orders of magnitude.

KEY POINT 7.6

Traditional machining methods utilize subtractive manufacturing—​material is

removed to produce the final product, for example, a hole is drilled into a metal plate,

and a lathe is used to generate a sharpened tip. Conversely, 3D printing is an example of

additive manufacturing, in which material is added together from smaller components

to generate the final product.

A 3D printer operates on the principle of additive manufacturing, in which successful

2D layers of material are laid down to assemble the final 3D product. Most commonly, the

method involves fused deposition modeling. Three-​dimensional objects can be first designed

computationally using a range of accepted file formats. A 3D printer will then lay down

successive layers of material—​liquid, powder, and paper can be used, but more common are

thermoplastics that can be extruded as a liquid from a heated printer nozzle and then fused/​

solidified on contact with the material layer beneath. These layers correspond to a cross-​

section of the 3D model, with a typical manufacturing time ranging from minutes up to a few

days, depending on the complexity of the model.

The spatial resolution of a typical 3D printer is ~25–​100 μm. However, some high-​

resolution systems can print down to ~10 μm resolution. Several cost-​effective desktop 3D

printers cost, at the time of writing, less than $1000, which can generate objects of several

tens of centimeter length scale. More expensive printers exist that can generate single printed

objects of a few meters in length scale. Cheaper potential solutions exist generating large