This document discusses nanowire-based biosensors and their applications in biomedicine. It begins by defining sensors and biosensors. Nanowires can be used as sensors due to their distinct electrical and optical properties. There are two main types of nanosensors: mechanical and chemical. Nanowire field-effect transistors can detect charged biomolecules through changes in conductivity. These sensors have been used to detect proteins, DNA, RNA and viruses by functionalizing the nanowire surface with specific receptors. While promising for detection, further improvements are needed for commercialization, such as enhanced sensitivity and simpler fabrication processes.

1. Nanowire-Based Biosensors and
their Biomedical Applications.
Lone Fawad Majeed
3rd Semester
20090119003
Department of Nanotechnology,
University of Kashmir.

2. INTRODUCTION
What is a sensor?
A sensor is a converter that measures a physical quantity and converts it into a
signal that can be read by an observer or by an instrument.
What is Biosensor?
A Biosensor is an analytical device that detects changes in Biological processes
and converts them into an electrical signal.
What are Nanosensors?
Nanosensors are any sensor used to convey information about nanoparticles to
the macroscopic world.

3. Types of Nanosensors
• There are two types of nanosensors –
• Mechanical Nanosensors; Nanomaterials that are used as mechanical
nanosensors change their electrical conductivity when the material is
physically manipulated, and this physical change invokes a detectable
response.
• Chemical Nanosensors; Nanosensors that detect chemicals work by
measuring the change in the electrical conductivity of the
nanomaterial once an analyte has been detected.

4. Nanowire Field-effect Sensors
• The primary task of biomedical sensors is to detect and characterize
chemical and biological species; from disease diagnosis to drug
discovery.
• Using nanomaterials with distinct optical, magnetic, and electrical
properties, those primary biomedical tasks can be achieved readily.
• Semiconductor nanowires enabled the detection of numerous species
electrically and without the use of labels.
• Their surfaces can easily be changed to make them sensitive to chemical
and biological species. (Surface Functionalization)

5. Working Principle
• Nanowire field-effect transistors (FETs) are a type of nanowire sensor
that have originated from the standard planar FETs which consist of a
gate, source, drain, and the body.

6. • The gate, a critically thin isolation layer (e.g., SiO2), is fabricated
between the source and drain and generates electric potential
variations to adjust the conductivity between the source and drain.
• These variations in electric potential are usually in response to the
application of an external voltage.
• In the nanowire FETs, doped channels and gates are replaced with
nanowires and receptors, respectively.

7. • Despite the differences in structure, the operating concept of nanowire
FETs is the same as that of ordinary FETs, in which the conductivity of
nanowires (i.e., doped channels) is affected by external changes generated
by charged species.
• Si nanowire is used because:-
Because of their great compatibility with standard CMOS (complementary
metal–oxide–semiconductor); natural growth of the oxide layer as an
isolation layer; ease with which the silicon and silica surface can be modified.
The charged biomolecule does not attach to the isolation layer on its own but
needs to be functionalized and bound by receptors for specific charged
species (e.g. DNA RNA etc.).

8. • The typical binding process using APTES, which is a type of receptor for DNA,
peptide nucleic acid(PNA), and antibodies. APTES converts silicon-oxygen bonds to
a silane chemistry layer (Si–O–Si–X), where X can be modified further or linked
with specific receptors.
• The receptors that bind the collected species then create an electric field on the
nanowires, changing their conductivity.
• As we consider species as an input signal, the receptors play a similar role to the
gate since they convert the input signal to changes in the conductivity of devices.

9. • On binding of charges either positive or negative, the opposite charge is
induced on the Si nanowire FET surface.
• The generation of positive charges can either be considered as the
disappearance of electrons or the appearance of holes, therefore changing
the conductivity between the source and the drain.
• In the n-type Si nanowire, the conductivity will decrease as the charge
carriers in this case are electrons. On the contrary, in p-type Si nanowires,
the current will increase as the charge carriers in p-type nanowires are
holes.
• A semiconductor parameter analyzer is used for the real-time monitoring
of conductivity from the Si nanowire FETs.

10. Biomedical Applications of Nanowire Sensors
DETECTION OF PROTEINS
• First application of p-type silicon nanowire sensors was to detect
proteins in a solution, electrically.
• This is itself based an antigen-antibody interaction, where antigens
are immobilized over the NW surface, and interaction is noted on the
basis of change in conductivity.
• Using this method, many different classes of proteins have been
detected as analytes. Prostrate detection in cancer and Troponin I in
cardiac disorders have already been demonstrated.

11. • Janissen et al. presented comprehensive work that characterizes the
effects of surface functionalization and for protein detection from
pathogens.
• Studied the surface functionalization with APTES (3-aminopropyl-
triethoxysilane), ethanolamine, was compared to the surface passivation
via poly(ethylene glycol) (PEG).
• It was found that, compared to APTES, applying ethanolamine significantly
promoted the bioreceptor density and coating homogeneity.
• In addition, applying PEG led to many benefits, including
• (1) higher specific ligand binding;
• (2) increased receptor/ligand binding based on spatial separation;
• (3) improved the antigen-binding.

12. DETECTION OF DNAAND RNA
• The surfaces of silicon nanowires consist of single-stranded sequences of
PNAs(peptide nucleic acid) that are placed to act as receptors for DNA.
• The capability of silicon nanowires to detect DNA at the 10 femtomolar
levels has been demonstrated. This is significantly better for DNA detection
as compared to the quartz-crystal microbalance, surface plasmon resonance
(SPR), and nanoparticle-enhanced SPR.

13. DETECTION OF VIRUSES
• In recent years, Si nanowire sensors have been used to detect many
dangerous viruses, including Dengue, influenza A H3N2, H1N1, and
HIV.
• The nanowire sensor surface is functionalized with antibodies that
specifically bind to the target viruses, affecting the conductivity of the
nanowire.
• For instance, Shen et al. developed a Si nanowire-based biosensor that
could detect as many as 29 flu viruses/µL from exhaled breath
condensate (EBC) samples.

14. Conclusion
• With specific receptors, nanowire sensors have shown great
potential to become a practical detection platform in biological
and medical applications.
• The present devices have several advantages, such as:- high
sensitivity and feasibility of label-free detection.
• In spite of these attractive features, some improvements are still
required for commercialization.
• Improvements in receptor binding methods could resolve this
higher sensitivity issue as well as more simple fabrication
processing issues.
• Nevertheless, the success of a nanowire sensor will depend on
how advanced it is compared with the current gold standards,
such as PCR and ELISA, in terms of simplicity, sensitivity,
specificity, and reliability.

15. THANK YOU.