Engr 1182.03

Presentation Short Title 1

College of EngineeringEngineering Education Innovation Center

Engr 1182 Nano Pre-Lab

Detection Circuit 2

Rev: 20XXMMDD, Initials

Engr 1182.03

Presentation Short Title 2

Wavelengths vs. Features Size

Rev: 20XXMMDD, Initials

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 10-4 10-6 10-8 10-10 10-12 Wavelength in meters

Nanotechnology

Engr 1182.03

Presentation Short Title 3

Fluorescein Detection

Rev: 20XXMMDD, Initials photodetectorblue LED

Acrylic chipdetection well

emis

sio

n

excitation

Looking at a side view where Acrylic chipis sitting on top of the DAD

Engr 1182.03

Presentation Short Title 4

Absorption of Light

Rev: 20XXMMDD, Initials

• Light of energy hnex is absorbed and excites an electron from E0 to E1 or to a higher energy level.

hnex Electronic Energy Levels

E1

E0

Engr 1182.03

Presentation Short Title 5

Relaxation and Emission

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.

E0

E1

hnem Electronic Energy Levels

Engr 1182.03

Presentation Short Title 6

Fluorescein

Rev: 20XXMMDD, Initials

Emission will ALWAYS be at a longer wavelength than the excitation.

Blue spectrum

Green spectrum

Engr 1182.03

Presentation Short Title 7

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

Engr 1182.03

Presentation Short Title 8

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

Engr 1182.03

Presentation Short Title 9

Detection Alignment Device (DAD)

Rev: 20XXMMDD, Initials

DetectorSlot LED

Chamber

Chip HolderAlignment

Holes

Engr 1182.03

Presentation Short Title 10

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.

Rev: 20XXMMDD, Initials

Plexiglass

PlexiglassPDMS Detection well

Detector aperture

LED

ReflectedLight

DADAcrylic

Engr 1182.03

Presentation Short Title 11

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

Engr 1182.03

Presentation Short Title 12

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