Biological Sensing Biological Sensing via via THz Circular Dichroism THz Circular Dichroism http://www.isa isa.au. .au.dk dk/SR/UV1/ /SR/UV1/cd cd- -spectroscopy.html spectroscopy.html http://www.
WHAT??? WHAT??? • We shine light through stuff… We shine light through stuff… • http://school.discovery.com/clipart/clip/flashlte http://school.discovery.com/clipart/clip/ flashlte.html .html http://www.astbury http://www. astbury. .leeds leeds.ac. .ac.uk uk/gallery/ /gallery/tly tly.gif .gif
Still to Come Still to Come • Background Background • – Light as an Electric Field Light as an Electric Field – – Polarized Light Polarized Light – – Chirality Chirality – – Circular Dichroism – Circular Dichroism • The Experiment • The Experiment – Biological Sensing – Why Circular Dichroism? – Why Terahertz? – Research Approach – Developing the Spectrometer – Conclusion • Acknowledgements
Background
Light as an Electric Field • The human eye detects electromagnetic waves (a moving The human eye detects electromagnetic waves (a moving • electric field) and interprets wave frequency and amplitude electric field) and interprets wave frequency and amplitude as color and intensity. as color and intensity. http://www.jb http://www. jb.man.ac. .man.ac.uk uk/distance/ /distance/strobel strobel/light/ /light/lighta lighta. .htm htm
Polarized Light • Linearly Polarized Light The electric field exists in a constant plane The electric field exists in a constant plane , while , while changing magnitude changing magnitude periodically. periodically.
Polarized Light • Circularly Polarized Light The electric field remains constant in magnitude The electric field remains constant in magnitude , while , while changing direction changing direction periodically. periodically.
Chirality • A chiral object has no plane of symmetry. http://www.answersingenesis http://www. answersingenesis.org/docs/3992.asp .org/docs/3992.asp • An achiral object has a plane of symmetry. http://cwx http:// cwx. .prenhall prenhall.com/ .com/bookbind bookbind/ /pubbooks pubbooks/bruice2/chapter4/deluxe.html /bruice2/chapter4/deluxe.html
Circular Dichroism • Chiral (asymmetric) objects are unequally sensitive to left and right circularly polarized light. • Circular dichroism is a term that describes this sensitivity. http://www.isa http://www. isa.au. .au.dk dk/SR/UV1/ /SR/UV1/cd cd- -spectroscopy.html spectroscopy.html • An object that absorbs the two circular polarizations differently will give a CD signal.
The Experiment
Biological Sensing Biological Sensing • Must distinguish between biological and abiological materials – Biological materials exist in enantiomeric excess (a single handedness is dominant). – Abiological materials exist in enantiomeric equality. • Species specific characterization – Each biological material exhibits unique spectral features. –These spectral features provide a potential “fingerprint” for biopolymers.
Why Circular Circular Dichroism Dichroism? ? Why • Sensitive to net left versus right sample chirality – CD could provide life detection! Biological systems Non- -Biological systems Biological systems Biological systems Non 100% 0% 50% 50% http://cwx cwx. .prenhall prenhall.com/ .com/bookbind bookbind/ /pubbooks pubbooks/bruice2/chapter4/deluxe. /bruice2/chapter4/deluxe. http:// html html • Eliminates artifacts and background absorbances of non- biological materials. – Allows for potential species specific spectral analysis in the presence of non-biological absorbers.
Why Terahertz? • Visible light stimulates electronic transitions that are symmetric (yield no net CD signal). – These transitions respond equally to left and right circular polarizations. • THz radiation tends to excite overall vibrational modes. – Dynamic modes of oscillation respond differently to left versus right circular polarizations. Visible light shakes bonds , THz shakes the entire molecule .
Research Approach • Before we can investigate the THz spectral features of biological specimen using CD we must first develop the spectrometer. First unsuccessful optical setup. Parabolic Mirror Reference Pyroelectric Detector Detector Reference Beam Parabolic Mirror Beam Splitter Iris Sample holder Gunn Collimator Lens Quartz WavePlate Oscillator
Developing the Spectrometer • First unsuccessful optical setup. – Focusing lens too thick. – Undesirable waveplate behavior. Intensity vs. Position (along beam axis y=k) with Collimator Source Horn close to Wave Plate 0.6 0.55 intensity (mV) 5.0 0.5 0.45 Transmission (mV) 4.5 0.4 90 135 0.35 4.0 180 225 0.3 270 0.25 315 3.5 0 0.2 45 6 8 10 12 14 16 18 3.0 position (cm) 2.5 0 20 40 60 80 100 98 Independent Measurements Interpretation Source Horn close to Wave Plate 5 Average Transmission 4.5 (mV), 98 events 4 3.5 Gunn Oscillator 3 Collimator lens 2.5 0 90 180 270 360 Wave Plate Orientation (deg) •Close to the lens the beam converges accounting for the increasing intensity. •n*90 degrees, for n = 0, 1, 2, … represent the optical axes of the waveplate. •Farther away from the lens the beam •Minimizing the difference in transmission between the axes of the quartz is diverges accounting for decreasing intensity. crucial to the quality of circular polarization produced.
Developing the Spectrometer • We are currently using this experimental setup to examine the relationship between source, waveplate, and detector. Gunn Quartz WavePlate Pyroelectric Detector Oscillator • The LabVIEW programming environment is used to collect and process data.
Conclusion • We will next explore several methods to improve waveplate performance, such as: – Introducing a small tilt in the plates orientation. – This will alter the path length of the beam, potentially reducing the deconstructive interference due to internal reflections. – Evaporate specialized thin films to the plates surface. – These films are designed to reduce reflections. • Alternative methods to generate circular polarization may need to be employed if we are unable to sufficiently minimize the difference in transmission between the waveplate axes. • More experiments are needed to discover the optimal experimental setup.
Acknowledgements
Primary Investigators • Kevin Kevin Plaxco Plaxco • –University of California at Santa Barbara Department of Chemistr University of California at Santa Barbara Department of Chemistry y – and Biochemistry and Biochemistry • Gerald Gerald Ramian Ramian • –University of California at Santa Barbara Institute for Quantum – University of California at Santa Barbara Institute for Quantum Engineering, Science and Technology Engineering, Science and Technology • S. James Allen S. James Allen • –Institute for Quantum Engineering, Science and Technology Institute for Quantum Engineering, Science and Technology – –University of California at Santa Barbara Physics Department University of California at Santa Barbara Physics Department – • Funding Funding • –NASA NASA – –United States Army United States Army –
•RET Interns –V. Chanyavanich –Ralph Reid •UCSB Physics graduate student –Jing Xu •INSET Interns INSET Interns • –Matthew Matthew Crossley Crossley – –Ventura College, UC Santa Barbara Ventura College, UC Santa Barbara – –Frances Ho Frances Ho – –West Valley College West Valley College –
THANK YOU YOU THANK THE END http://school.discovery.com/clipart/clip/flashlte http://school.discovery.com/clipart/clip/ flashlte.html .html
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