Fabrication and derivatisation of nanosensors. Their applications in nanomedicine.

Melerin Madekufamba

Introduction• Nanotechnology opened doors to new

ways of identifying and quantifying biomolecules through use of nanosensors and nanoprobes.

• Tools are capable of monitoring biomolecular processes within single cells.

• Use in biological and medicinal research.• Two major categories, (i) biosensors

(consists of biological recognition element called bioreceptor (Ab, NA or enzyme, cell) and a signal transducer (ii) chemical sensors (chemical recognition element eg pH sensitive dye) and a signal transducer).

T Vo-Dinh et al; Sensors and Actuators; B 74 (2001) 2-11

BiosensorsWhen target analyte interacts with the bioreceptor the resulting complex produces a change (physicochemical perturbation) which is converted to a measurable signal by the transducer.Some complex reactions are:(Ab/Ag interaction, NA interactions, Enzymatic reaction etc).

Most prevalent signal transduction methods are:Optical measurements( luminescence, absorption)Electrochemical (potentiometric, amperometric).Mass sensitive techniques eg surface acoustic wave and microcantilever).

Nanofibers from inorganic materials as biosensors

Fabrication of NanofibersDerivatisationApplications

Fabrication procedure• Fabrication technique is a crucial

prerequisite for successful development and application. Two fabrication techniques

• Heat and pull and chemical etching.• Most developed fabrication technique is the

“heat and pull technique”.• Technique involves heating (with laser or

heat filament) and pulling nanotips from larger diameter (~600:M) silica optical fiber.

• First the fiber is secured into the pulling device.

• Heating is centered to the median of the fiber.

Fiber optic puller

Capable of pulling aluminosilicate and borosilicate glassProgrammable (heat and filament characteristicsIts integrated with a CO2 laser base heat source.Pulls tips up to diameter of 20nm.Can produce even heating on original fiber of up to 1.2mm

http://spectraservices.american-data.net

Sutter instrument P-2000

Example of the SEM image of the distal end of the nanofiber tip.

Once tip dimensions are right the next stageis coating the tapered end of fiber with a thin layer of silver, aluminum, or gold.Method is thermal vapor deposition while leaving the distal end uncoated. Once thermal vapor depositionthe fiber is secured in a thermal evaporation chamberto ensure uniform silver coatingCoating restores refractive index and enablespropagation of excitation light down the tapered sidesof the fiber.

Functionalisation

objective is to make the distal end of the nanofiber to biorecognise certain molecules or substance (specificity). This is done by exposing tip end to chemical or molecule that can covalently bind to tip.Material depends on particular application and method of detection to be employed.

Pulling fiber

Coating by vapour deposition living tip

derivatisation of fiber

Antibody binding

(a) Glycidoxypropyltrimethoxy silane (GOPS) followed by 1,1 Carbonyldiimidazole (CDI): This treatment allows covalent binding of antibodies or synthetic peptides coupled to a fluorescent molecular probe, enabling biorecognition of molecules such antigens. (b) Antibody-based fiber optic biosensors were first used to detect benzo(a)pyrene tetro (BPT) a biomarker for human exposure to the known carcinogen benzo(a)pyrene) (Cullum et al 2000; T. Vo-Dinh 2001).

Examples of substances that have been used for tip derivatisation and functionisation.

B.M Cullum; Anal Biochem 277 (2000) 25-32

Most fiber-optic nanobiosensors are used with fluorescence spectroscopy.

• When the nanofibers have been covalently bonded to Ab or peptides that are coupled to flourescent molecular probes then fluorescence technique can be used for detection.

• Light source is used to excite the molecule. Because the tip is so small photon travel down the fiber providing excitation for the flourescent species of interest present in the vicinity of the biosensing layer for example only antigens binding to the antibodies can be excited.

Schematic of optical measurement system

B.M Cullum and T.Vo-Dinh; TIBTECH; 18 (2000) 388-393

NB diameter of the tip is significantly less than λ of light exciting analyte.

Fiber Optic Chemical nanosensors

• Nanofibers are also used in chemical sensing.

• Possible to probe specific chemicals in localized area this allows monitoring of concentration gradients, spatial inhomogeneity in submicroscopic environments (cell) using spectroscopic techniques. Fluorescence is a commonly used technique in optical chemical sensing techniques. Its been used for trace analysis.

Potential Applications in biological and medical research.

• Offer ability to observe processes within living cells.

• Optical nature of detection signal means can be used for in-situ monitoring applications with no interference with cell surface potentials.

• The small size of nanofibers allows the sensing of intracellular /intercellular physiological and biological parameters.

• Biomedical diagonisis e.g for cancer cells can be made possible, toxic elements .

Fiber optic nanobiosensors for monitoring apoptosis.• Defn: Process by which cells in our tissues

and organs degenerate during normal development, aging or in disease. Malfunction of apoptosis can lead to cancer, hydrocephalus and neurodegenerative disease.

• Fiber optic biosensors are an important tool for monitoring apotosis proteins early in cell death cascade. One area of current application is in monitoring of protein MCF-7 a human mammary carcinoma cell known to cause cancer in woman. This is widely used in breast cancer research.

Applications in nanomolecular imaging in medical research

Imaging techniques rely on radio-labeled nanoparticles (eg liposomal nanoparticles) to deliver radiation doses.When nanoparticles are introduced to an organ they target specific cells (diseased cells). Imaging techniques are employed to detect the targeted cells eg:

Fluorescence images showing diseased lung cells mapped by nanoparticle.

• Nanoparticles used as probes in diagnosis of disease cells.

Nanoparticles used in imaging

Nanoparticles used to map tumor targets

Conclusions

1: The fast growing field of nanotechnology is contributing tremendously to medical research (nanomedicine).

2: Diverse applications:(i) detection of exposure to toxins or carcinogens.(ii) quantification in cellular metabolism, (ii) monitoring diseased cells/tissues to diagnosis.

Fiber-optic nanosensors and other nanoparticles sensors• The small size of the fiber tip is the

major factor contributing to their use in nanomedicine.

• The optical florescence technique which is highly sensitive analytical tool works well with these types of sensors.

• There is still a lot of research going on in the development of fiber-optic nanosensors.