For more than a century, Kodak (www.kodak.com/go/specialtychemicals) has been creating new ideas, then finding chemical processes to bring them to life. One of those product innovations – Indocyanine Green (ICG), a fluorescent dye – was invented and patented by Kodak in the 1950’s.
After receiving FDA approval in 1959, ICG was initially used in liver function diagnostics and later in cardiology. In the years since 1980, the development of new types of cameras, better film material and new photometric measuring devices has cleared away many of the dye’s earlier technical difficulties – and opened the door to additional applications.
Today ICG still is the only near infrared (NIR) dye approved by the FDA for clinical applications. It is administered intravenously and, depending on liver performance, is eliminated from the body with a half-life of approximately three to four minutes.
Optical Properties of NIR dyes
Near infrared (NIR) dyes in the Indocyanine class – of which ICG is one – can exhibit fluorescence in the near infrared range (700-1000nm). They are effective in situations where low background autofluorescence, low light-scattering and higher signal-to-noise ratios are critical – and only when the dye’s structure is rigid and in an “excited” state – otherwise the energy created will be released as heat through molecular rotation and vibration.
The light needed to “excite” the fluorescence is generated by a near infrared light source which is attached directly to a camera. A digital video camera allows the absorption of the ICG fluorescence to be recorded in real time, so the body tissue in question can be assessed and documented.
Proper design of the dye structure can influence the depth of the dye’s absorption as well as its organic or water solubility. For instance, adding a heteroatom to the center of the heptamethine chain can deepen the lambda max by as much as 90nm (lambda max of 875nm). Shortening the heptamethine to a pentamethine, will reduce the absorption max by as much as 100nm (lamda max around 680nm).
Similarly, changing from a napthindole to an indole will shorten the absorption max by 40nm (lambda max around 741nm). Likewise, by adding multiple sulfonate groups you can control the water solubility.
The maximum wavelength for fluorescent emission often is approximately 30nm longer than the absorption maximum.
Diagnostic applications of NIR dyes
For biological imaging, fluorescent indole heptamethine cyanine dyes have found multiple useful applications – from navigating breast cancer sentinel lymph node removal, to guiding cardiovascular, gastroenterological, plastic and reconstructive surgery.
Due to its solubility, the dye partially binds to lipoprotein in the blood. Because of this, the substance serves as a parameter for monitoring the progress toward recovery of post- surgical patients, by measuring the body’s blood circulation and clearance behaviors.
For “in-vivo” applications (i.e. within living organisms, such as laboratory mice), there is a “window of imaging” limited by the absorption of hemoglobin (below 650nm) and water (above 900nm). ICG can be excited with light in the 750-800nm range and fluorescence can be detected beyond 800nm using infrared camera systems designed specifically for this application.
NIR dyes in security applications
This feature may also provide powerful design criteria for next generation optical probes beyond bio-imaging to security applications as well. Fluorescence in the visible region is becoming quite a common feature, and specialty inks that can offer this are becoming quite common today.
However fluorescence in the IR region requires unique equipment that can separate the absorbance spectra from the fluorescence spectra.
Another interesting application is FRET (for Fluorescence Resonance Energy Transfer). FRET is a mechanism that enables scientists to obtain larger separations between the excitation and emission of chromophores – that part of the molecule that is responsible for its color. The color arises when a molecule absorbs certain wavelengths of visible light and transmits or reflects others.
In FRET, two or more fluorescent dyes are combined in which the emission from a lower wavelength “donor dye” is absorbed by a longer wavelength “accepter dye,” allowing unique and greater separation of the observed emission.
Summary
Classes of infrared dyes like indocyanine commonly exhibit fluorescence; other classes like phthalocyanine, benzothiazole cyanines, merocyanines, and diiminium, unexpectedly do not. For more information about Kodak functional and IR dyes, call visit Nagraj Bokinkere, Technical Director, Kodak Specialty Chemicals, at nagraj.bokinkere@kodak.com (tel. +1 585-722-9038), or visit www.kodak.com/go/specialtychemicals.