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Fri, 21 Nov 2003 20:27:49 GMT

Metal Nanoshells, Cure or Curse?

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The Institute of Science in Society

Science Society Sustainability

http://www.i-sis.org.uk

 

General Enquiries sam

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ISIS Director m.w.ho

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Metal Nanoshells, Cure or Curse?

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Among the nanoparticles developed for use in medical and other applications are

non-biodegradable metal nanoshells. Has enthusiasm to exploit their remarkable

properties run too far ahead of safety considerations? Dr. Mae-Wan Ho reports.

 

 

 

Metal nanoshells are a class of nanoparticles with tunable resonance to

electromagnetic radiation. They consist of a spherical dielectric core

nanoparticle, such as silica, surrounded by a thin metal shell, such as gold.

 

 

These particles possess a highly tunable ‘plasmon resonance’, whereby light of

particular frequencies causes collective oscillations of conductive metal

electrons at the nanoshell surface, thus greatly concentrating the intensity of

the light. Whereas many bulk metals demonstrate plasmon resonance behaviour,

they do so generally over a very small region of the visible spectrum.

 

 

In nanoshells, however, their plasmon resonance can readily be tuned to a wide

range of specific frequencies, from the near ultra violet to the mid-infra-red,

simply by controlling the relative thickness of the core and shell layers of the

nanoparticle. This range spans the near infrared, a region where absorption in

tissue is minimal and penetration is optimal.

 

 

To date, nanoshells have demonstrated their usefulness in many applications

ranging from inhibition of photo-oxidation in photoluminescent polymer films to

biosensing and light-triggered drug delivery.

 

 

One possible application is in removing diseased tissues without complicated

surgery. Recently, lasers, microwaves, radiofrequency radiation, and focussed

ultrasound, have all been used to heat up and kill diseased tissues selectively

without invasive surgical procedures. But these can still cause damage to

intervening tissues.

 

 

Researchers in Rice University Texas USA thought that by tuning nanoshells to

strongly absorb light in the near infrared, where optical transmission through

tissue is optimal, nanoshells embedded in tissues can be used to deliver a

therapeutic dose of heat to the tissues by moderately low levels of light

applied outside the body.

 

 

In a paper just published in the PNAS (house journal of the US National Academy

of Sciences), the research team reported that human breast carcinoma cells in

culture incubated with nanoshells died when exposed to near infrared light

(820nm, 35W/cm2) while control cells not containing nanoshells appeared

unharmed.

 

 

Similarly, in live animals with solid tumours into which metal nanoshells were

injected, exposure to near infrared light (820nm, 4W/cm2) caused the tumours to

heat up by some 400C, while controls without nanoshells heated up by less than

100C. Cells in tissues heated above the thermal-damage threshold were killed,

while control tissues appeared undamaged.

 

 

The gold surface of the nanoshell can also catalyse the self-assembly of

polyethylene glycol, antibodies, or a variety of other agents. This offers the

potential to target the nanoshells to specific diseased tissues.

 

 

But are the nanoshells safe? They are non-biodegradable and have enhanced

catalytic capabilities. What happens to the nanoshells in the dead cells when

they are cleared by the immune system? What effects do they have on the health

of the patient in the long term? What are the wider environmental impacts when

these nanoshells are discharged or released? None of these questions have been

addressed.

 

 

It is clear that enthusiasm to exploit the remarkable properties of metal

nanoshells and other nanoparticles have run far ahead of any safety concerns. It

is time for responsible scientists to impose a moratorium on research and

development until proper safeguards are put in place.

 

 

 

 

Source

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Hirch LR, Stafford RJ, Bankson JA, Sershen SR, Rivera B, Price RE, Hazle JD,

Halas NJ and West JL. Nanoshell-mediated near-infrared thermal therapy of tumors

under magnetic resonance guidance. PNAS 2003, 100, 13549-54.

 

 

 

 

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General Enquiries sam

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ISIS Director m.w.ho

 

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