Biomedical Engineering Reference
In-Depth Information
display high absorption coe
cient in the near-infrared and enable high local-
ization of power deposition, ideally down to the scale of individual biological
structures. Further desirable features include good chemical and thermal sta-
bility and high photo-bleaching threshold.
Conventional chromophores of common use in laser-welding are organic
molecules such as ICG. These have given outstanding experimental and clini-
cal achievements in a number of medical fields, and that in spite of relatively
poor performances with respect to the aforementioned criteria [102,103]. The
absorption e
ciency and photo-stability of organic molecules are limited.
Their optical properties depend strongly on biochemical environment and
temperature, and generally deteriorate rapidly with time [104]. The range
of chemical functionalities accessible is narrow, which is incompatible with
flexible and selective targeting of distinct biological structures. Overcoming
of these limitations would represent a real breakthrough in the practice of
laser-welding.
Possible ways forward are disclosed by the advent of nanotechnology, as
a powerful paradigm to develop new functionalities, by manipulation of self-
organization processes at the nanoscale. Here, we mention the introduction
of a new class of nanostructured chromophores, which is attracting much
attention in view of many applications, including the laser-welding of bio-
logical tissues: colloidal gold nanoparticles (nano-gold). Whereas the opti-
cal response of organic molecules stems from electronic transitions between
molecular states, light absorption, and scattering in nano-gold originates from
excitation of collective oscillations of mobile electrons, i.e., surface plasmon
resonances [102]. This translates into molar extinction coe
cients higher by
4-5 orders of magnitude with respect to those of organic chromophores [102],
enhanced thermal stability and photo-bleaching threshold, lower dependence
on surrounding chemical environment (although surface plasmons shift with
variations in dielectric constant and electron donating/withdrawing tendency
of embedding tissues [105, 106]). Inspiring perspectives arise from the surface
chemistry of nano-gold. As a traditional material for implants, gold is be-
lieved to ensure good biocompatibility [107], which is a critical prerequisite
in front of clinical applications. The possibility of flexible conjugation of gold
surfaces with biochemical functionalities opens a wealth of novel opportuni-
ties [108], such as selective targeting against desired and well-defined biological
structures. In summary nano-gold may become the ideal substitute of organic
chromophores.
The utilization of nano-gold dates back to the ancient Romans, when
employed for decorative purposes in the staining of glass artifacts (e.g., the
Lycurgus cup). Synthesis of stable aqueous colloidal preparations of nano-gold
was first achieved by M. Faraday toward the 1850s by use of phosphorous to
reduce a solution of gold chloride. Subsequent developments led to a number
of variants especially based on reduction of chloroauric acid in sodium cit-
rate (Turkevich method) [109, 110]. Conventional nano-gold is composed of
spherical nanoparticles of variable and controllable radii [110]. Their optical
