Engr 1182.03

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Transcript Engr 1182.03

Engr 1182.03
Engr 1182 Nano Pre-Lab
Detection Circuit 2
College of Engineering
Engineering Education Innovation Center
Rev: 20XXMMDD, Initials
Presentation Short Title
1
Engr 1182.03
Wavelengths vs. Features Size
The wavelength of a sinusoidal wave is the spatial period of the wave— the
distance over which the wave's shape repeats. The smaller the wavelength of the
wave the higher the energy of the wave.
100 10
10-2
Wavelength in meters
10-4
10-6
10-8
Presentation Short Title
Nanotechnology
Rev: 20XXMMDD, Initials
10-10
10-12
2
Engr 1182.03
Fluorescein Detection
detection well
Acrylic chip
Looking at a side view where Acrylic chip
is sitting on top of the DAD
blue LED
Rev: 20XXMMDD, Initials
photodetector
Presentation Short Title
3
Engr 1182.03
Absorption of Light
E1
hnex
Electronic
Energy Levels
• Light of energy hnex
is absorbed and
excites an electron
from E0 to E1 or to a
higher energy level.
E0
Rev: 20XXMMDD, Initials
Presentation Short Title
4
Engr 1182.03
Relaxation and Emission
E1
hnem Electronic
Energy Levels
E0
Rev: 20XXMMDD, Initials
• Excited electron
"relaxes" to the E1 state
and then gets back to E0
by “emission” (releasing)
of energy
– E1-E0 =hnem< hne
• This energy of emission
is in the form of light.
Presentation Short Title
5
Engr 1182.03
Fluorescein
Emission will ALWAYS
be at a longer
wavelength than the
excitation.
Blue spectrum
Green spectrum
Rev: 20XXMMDD, Initials
Presentation Short Title
6
Engr 1182.03
Signal to Noise Ratio (SNR)
• In any sensor circuit we must be concerned with
detecting unwanted signals which would invalidate
our measurements.
• These unwanted signals are called “noise.”
• A figure of merit of a sensor circuit design is the
relative strength of true signal divided by
unwanted signal detected: signal-to-noise ratio.
• The higher the SNR the better the performance of
the sensor circuit we have.
Rev: 20XXMMDD, Initials
Presentation Short Title
7
Engr 1182.03
Optical Considerations for SNR
• Alignment of the LED and detector is very
important for a high quality sensor circuit.
• Reflection off internal surfaces needs to be
minimized
• Outside excitation sources must be shielded
from the photodetector.
Rev: 20XXMMDD, Initials
Presentation Short Title
8
Engr 1182.03
Detection Alignment Device (DAD)
Detector
Slot
Rev: 20XXMMDD, Initials
Chip Holder
Alignment
Holes
Presentation Short Title
LED
Chamber
9
Engr 1182.03
SNR: LED / Detector alignment
• A critical angle of blue LED is chosen such that it maximizes
the exposure of detection well to the blue LED excitation so
that photodetector senses green emission from the
fluorescein in the detection well only.
Plexiglass
PDMS
Detection well
Reflected
Light
Acrylic
Plexiglass
Detector aperture
LED
Rev: 20XXMMDD, Initials
DAD
Presentation Short Title
10
Engr 1182.03
SNR: Reflections from internal surfaces
• Since we have a stacked layer of materials to assemble our LOC,
surface of the each layer can act as a reflecting surface.
• Reflections occur at those surfaces and they erroneously increase
the signal seen by the detector.
• Those reflections can be minimized by no air pockets between
surfaces(layers).
• Detection well should be fully filled with fluorescein so that no air
gaps remain.
• Keep surfaces clean and smooth to avoid reflections.
Rev: 20XXMMDD, Initials
Presentation Short Title
11
Engr 1182.03
SNR:Outside excitation sources
• We are interested in only the signal from the direct
emission of fluorescein.
• Room light contains all wavelengths and will create
significant error.
• Notice the DAD uses a lot of black.
• Black absorbs light and reduces reflection
Rev: 20XXMMDD, Initials
Presentation Short Title
12