Electromagnetic Spectrum and Its Absorption by Molecules
X-rays UV/Vis IR Radio
Near Mid Far
HighEnergy
LowEnergy
200
800
2500
2500
0Wavelength (nm)
Ee Ev Er EnsCore Bonding/
Valence
Mol
ecul
arE
nerg
y ty
pes
Light is quantizedE= hc/λ
Energy levels in molecules are quantized
Elight = ΔEEmoleclues (between energy levels) → absorbance or emission
4000
400
Wavenumbers, cm-1
Review of IR Spectroscopy (a kind of vibrational spectroscopy)Rules for Infrared Spectroscopy
Vibrational Spectroscopy
● Not vibrating light● Vibrating molecules
http://www.chemeddl.org/resources/models360/models.php?pubchem=313
https://www.chemtube3d.com/spectrovibhcl1-ce-final/#models
http://sites.science.oregonstate.edu/~gablek/CH335/Chapter10/IR_vibrations.htm
Using light to probe of vibrational modes of molecules (molecular ions)
Spectroscopy is probing atoms, molecules, ions with light
Scattering of Light
Jablonski Diagram
Ground electronic state
Electronic excited state
v=1v=2v=3
NIRAbsorption
IR (Mid)Absorption
RayleighScattering
RamanScattering
ResonanceRaman
Scattering
hν0 hν0
hν0 hν0 – hν1
hν0 hν0 – hν1
Virtualstate
Ramanhν0 = energy of excitation lighthν1 = energy of scattered light
Rayleighhν0 = energy of excitation lighthν0 = energy of scattered light
Stokes vs. antiStokes
Ground electronic state
Electronic excited state
v=1v=2v=3
NIRAbsorption
IR (Mid)Absorption
RayleighScattering
StokesRaman
Scattering
AntistokesRaman
Scattering
hν0 hν0
hν0
hν0 – hν1
Virtualstate
hν0 – hν1
hν0
Laser line
Measured data to Raman spectra
Wavelength /nm
Convert to Raman Spectrum(convention)
Wavenumber /cm-1
Example Raman Spectrum
Laser 785 nm
Selection Rules
+
+
+++
+
+
++
----------
e-e-
e-e-
δ+e-
e-e-
e-
δ+
e-e-
e-e-
e-e-
e- e-
δ+
Rule of Savannah: More electrons in the valence orbitals/involved in bonding, the more polarizable
Polarizability (α): The ability to form instantaneous dipoles ): The ability to form instantaneous dipoles
e-e-
e-e-
e-
e- e-
e-
Induced dipole
Selection Rule:Changing α): The ability to form instantaneous dipoles during vibration is key to strong signal.
Selection RulesRule of Mutual Exclusion
https://science.widener.edu/svb/ftir/ir_co2.html
Raman spectrum of CO2
IR spectrum of CO2
Center of inversion
Vibrational Modes3N-5 = 4
Intensity of Raman Scattered lightConcentration Dependence
Notice how Raman Intensity is not a Ratio of light through sample over light though blank (used in absorbance spectroscopy)● Intensity of laser correlates with intensity
of Raman signal (if your laser intensity is fluctuating then your Raman scattered light is fluctuating)● This would be the same problem with
Fluorescence● Intensity of Raman scattered light is laser
intensity dependent● Solution is often an internal standard
● Add a known amount of a chemical that does not chemically or optically interfere with the analyte to all standards
Relative Intensities of Raman Scattering
Type Iscattered/emitted --------
IincidentFluorescence 1 – 0.01Rayleigh 10-4
Stokes Raman 10-7
AntiStokes Raman 10-9
Resonance Raman 10-2 - 10-4
SE Raman > 1
Given a specific molecular system, what can you control?
Io and λ
Scattering Cross-Section
Process Cross-section of σ (cm2)
absorption UV 10-18
absorption IR 10-21
emission Fluorescence 10-19
scattering Rayleigh 10-26
scattering Raman 10-29
scattering Resonance Raman
10-24
scattering SERS 10-16
scattering SERRS 10-15
Conceptual construct:Area presented by the molecule for the process
Useful for comparing processes
Bigger the cross-section more likely the process
Take home message: Raman is a very, very, very unlikely process to occur.
Electronic excited state
v=1v=2v=3
RayleighScattering
hν0 hν0
hν0
hν0 – hν1
Virtualstate
hν0 – hν1
hν0
Vibrational relaxation
Fluorescence emissionStokesRaman
Scattering
AntistokesRaman
Scattering
● Rayleigh scattered light is same narrow bandwidth as laser.
● All molecules that Raman scatter very narrow band shifted from laser λ
● Fluorescence emission bandwidth is very broad
● Each molecule can have a slightly different wavelength
From McCreery 2000 p 4
Fluorescence interference almost always out “shines” Raman scattered light
samplewavelength
selectorand Rayleigh light
rejection
detector
LAS
ER Why this geometry?
Laser transmssion (if using a cuvette/transparent sample holder)
Laser cleanup filterNotch or edge filterRayleigh rejection filter
Sample handling
Raman can measure gases, liquids and solids. Close and far.
Fiber coupled Probe system
Raman Microscope
cocaine
Ways to enhance the Raman Signal1. make Raman signal taller2. make the fluorescence signal lower
Time-gated Raman or Lifetime filtering
based on the different time-dependence of fluorescence and Raman scattering signals upon a short-pulse optical excitation
Pulsed laser
process lifetime
fluorescence 100 ps - ns
Raman “instantaneous” fs - ps
Time difference allows for separating (filtering) the two types of light.
Uses a non-linear optical material that switches that rotates the light that passes through.
Filters allows only one polarization of light to pass
Transmission properties change with very high intensity light https://doi.org/10.1039/C9CP02430A
Time-gated Raman
DOI:10.1364/OE.21.031632
● Fluorescence intensity changes (x and y) when you change the laser excitation wavelength.
● Raman does not.
https://agora.cs.wcu.edu/~huffman/ia.html#lumin
Change excitation Wavelength
Shift excitation Raman difference spectroscopy (SERDS)
Uses a tunable laser (Ti:sapphire, most common) to take 2 Raman spectra with 2 different excitation wavelengths (νlaser = 785 nm and νlaser = 784 nm)Principle: fluorescence does not change that much with such a small excitation shift, but Raman bands shift by 1 nm (very predictable )
Bleaching● What is Bleaching?
● Chemical - ● Photo -
AntiStokes Raman
v=1v=2v=3
RayleighScattering
StokesRaman
Scattering
hν0 hν0
hν0
hν0 – hν1
Virtualstate
hν0 – hν1
hν0
AntiStokesRaman
Scattering
Fluorescences is only Stokes shifted
Resonance Raman
Electronic excited state
v=1v=2v=3
RayleighScattering
RamanScattering
ResonanceRaman
Scattering
hν0 hν0
hν0 hν0 – hν1
hν0 hν0 – hν1
Virtualstate
Enhancement of Raman Signal ● Enhancement factor (EF)● EF = 102 – 106● Additional Selection Rule
● Only vibrations coupled to electronic absorption are enhanced
Hamaguchi, 2013, Jena
Remember the requirements for fluorescence? π electrons
Substance λmax /nm
benzene 255naphthalene 286
anthracene 375
UV Raman
Improves● Intensity of Raman scattering● “suppresses” fluorescence
Problems:● Can not see the laser to align it.● Danger to human● Danger to sample
Nanotechnology: What is this?
Literal definition:● Technology – stuff that is useful to people● Nano – one the namometer scale (10-9 meters)
Working definition:Controlled creation, measurement, and understanding of matter in the range ~ 1 to 100 nm
Nanoscale Somewhat subjectiveor situational
What is the behavior?Why does it behave that way?
Instrumentation● Did you make it?● What are its
properties?
Think BIGsynthesis,● Bottom up● Top down
SynthesisMeasurement
Theory
Informationflow
Controlled Creation: assembly of molecules into nanostructures
Formation of Fe(CO)2 oneatom at a time
35 argon atoms spelling out “IBM”
Dip-pen nanolithography (DPN):
● “writing” of electronic circuits
● as well as patterning of biomaterials for cell biology research,
● nanoencryption
https://www.nanonics.co.il/solutions/nanolithography#videos_nano
Moore’s Lawthe density of transistors on an integrated chip (IC) doubling every 12 months (later amended to every 2 years).
Fullerenes and Carbon Nanotubes
C60
An array of aligned carbon nanotubes (scale bar 0.001 mm)
Schematic diagram showing a CNT-based top-gate FET with an ideal 5- to 10-nm CNT pitch. S, source; D, drain.
DOI: 10.1126/science.aba5980
Properties of Carbon Nanotubes● CNTs have high thermal conductivity● CNTs have high electrical conductivity● CNTs aspect ratio● CNTs are very elastic ~18% elongation to failure● CNTs have very high tensile strength● CNTs are highly flexible — can be bent considerably without damage● CNTs have a low thermal expansion coefficient● CNTs are good electron field emitters
https://en.wikipedia.org/wiki/Carbon_nanotube
Type of nanotube controls its property:Conductivesemiconductor
Surface-Rolling MoleculesAbstract: Design, syntheses, and testing of new, fullerene-wheeled single molecular nanomachines, namely,nanocars and nanotrucks, are presented. These nanovehicles are composed of three basic componentsthat include spherical fullerene wheels, freely rotating alkynyl axles, and a molecular chassis. The use ofspherical wheels based on C 60 and freely rotating axles based on alkynes permits directed nanoscalerolling of the molecular structure on gold surfaces. The rolling motion observed by STM resembles thesame motion performed by macroscopic entities in which rolling occurs perpendicular to the axles. A newsynthesis methodology, in situ ethynylation of fullerenes, was developed for the realization of the fullerene-wheeled molecular machines. Four generations of the fullerene-wheeled structures were developed, andthe latest fourth generation nanocar, 3b, along with three-wheeled triangular compounds, 4a and 4b, provideddefinitive evidence for fullerene-based wheel-like rolling motion, not stick-slip or sliding translation. Thestudies here underscore the ability to control directionality of motion in molecular-sized nanostructuresthrough precise molecular design and synthesis.
Examples of Nano machines
MCM-41 arrays
MCM-41: a solid of SiO2, a mesoporous material
“synthesis of MCM-41, surfactants (typically cetyltrimethylammonium bromide (CTAB)) are added to the synthesis solution. The surfactant initially forms rod-shaped micelles that eventually align together in hexagonal arrays. Silica species are added to cover the rods, and then calcination condenses the silanol groups, which bridges the silicon atoms with oxygen atoms. Ultimately, this organic template oxidizes and disappears, leaving behind fully-formed MCM-41.”
ordered and large pores with a diameter in the range 1.5-10 nm
Some Applications
● base material for platinum complexes used in luminescent oxygen sensing materials
● uptake of CO2 for gas purification
● surface functionalized MCM-41 a catalyst in the synthesis of bisphenol A.
Mesoporous materials
Nanoscale? Way below the defraction limit of light.
https://www.edmundoptics.com/knowledge-center/application-notes/imaging/diffraction-limit/
Good video describing defraction limitation of imaging (Just first few minutes)
Quantum Dots
Nanotechnology: What is this, just another word for chemistry? Sort of
localized surface plasmon resonance (LSPR)
Why Are Metals Shiny?
Plasmon: oscillation of electrons
To explain why metals (and graphite) are shiny, we invoke a combination of reflection, refraction, and the energy levels of MOs. When a photon of light is absorbed and reemitted, the electron moves from one orbital to another. Let us consider a piece of metal at room temperature. When a photon arrives at the metal’s surface it encounters the almost continuous band of MOs. Most photons, regardless of their wavelength, can be absorbed because there is an energy gap between orbitals corresponding to the energy of the photon. This process promotes electrons up to a higher energy level. As the electrons drop back down to a lower energy level, the photons are re-emitted, resulting in the characteristic metallic luster. Metals actually emit light, although this does not mean metals glow in the dark (like a light bulb or the Sun). Instead, metals absorb and re-emit photons, even at room temperature.
Why Are Metals Shiny?
https://chem.libretexts.org/Bookshelves/General_Chemistry/Book%3A_CLUE_(Cooper_and_Klymkowsky)/3%3A_Elements_Bonding_and_Physical_Properties/3.4%3A_Metals/Why_Are_Metals_Shiny%3F
Fig. 1 Study of absorption and scattering plasmonic optical properties of colloidal Ag NPs using UV-vis absorption spectroscopy. (A) Photos of colloids show: (a) light yellow; (b) yellow; (c) light orange; (d) orange-red; (e) red; (f) dark red; (g) purple; (h) purple violet; (i) violet; (j) blue; (k) light blue; and (l) green colors. (B) Normalized absorbance of UV-vis absorption spectra of the colloids of Ag NPs in (A) shows the peak wavelength (λmax) with FWHM at: (a) 393 nm (64 nm); (b) 405 nm (69 nm) with a weak shoulder peak at 526 nm; (c) 427 nm (110 nm) with a weak shoulder peak at 364 nm; (d) 461 nm (192 nm) with a weak shoulder peak at 382 nm; (e) 502 nm (160 nm) with two weak shoulder peaks at 422 and 342 nm; (f) 518 nm (146 nm) with two weak shoulder peaks at 430 and 340 nm; (g) 536 nm (158 nm) with two shoulder peaks at 418 and 340 nm; (h) 552 nm (166 nm) with two shoulder peaks at 414 and 340 nm; (i) 572 nm (172 nm) with two shoulder peaks at 416 and 338 nm; (j) 606 nm (212 nm) with a shoulder peak at 334 nm; (k) 646 nm (215 nm) with two shoulder peaks at 450 and 336 nm; and (l) 738 nm (130 nm) with two shoulder peaks at 420 nm (78 nm) and 330 nm.
https://doi.org/10.1039/C0JM01990A
Highly tunable color change that is a function of NP size
javascript:popupOBO('CMO:0000292','c0jm01990a')javascript:popupOBO('CMO:0000842','c0jm01990a')
Other peoples videos on Nano materials and Plasmonics
Surface plasmons https://www.youtube.com/watch?v=eVpxn5Cw6YM
Nanoplasmonics 1 https://www.youtube.com/watch?v=8iyShOidtYg8iyShOidtYgAbout SERS Nanoplasmonics 2 https://www.youtube.com/watch?v=iUyPssG9f_MNanoplasmonics 3 https://www.youtube.com/watch?v=s7iYjwYQmjM
Fiberprobe W/ sers https://www.youtube.com/watch?v=81rfymWn8jI
https://www.youtube.com/watch?v=eVpxn5Cw6YMhttps://www.youtube.com/watch?v=8iyShOidtYg8iyShOidtYghttps://www.youtube.com/watch?v=iUyPssG9f_Mhttps://www.youtube.com/watch?v=81rfymWn8jI
SERS: Electromagnetic Mechanism
hνL
hνL
AgNP
Plasmon forms in resonance with electric field of light
Optical cross-section is larger than NP
Example: SERS
Rhodamine 6G
633 nm laser (HeNe) 3mW
Hot spots: coupled resonances at the ‘gaps’ between metallic nano-objects.
Hot spot
The trick it to reproducibly ● create arrays of nanostructures● Get the analytes inside of them
SERS: Overcoming fluorescence with surface quenching
v=1v=2v=3
hν0
Vibrational relaxation
Fluorescence emission
Metal surfaceAdds new relaxation mechanism (no radiative)
SERS selection rules
Application of SERS to molecular orientation on surfaces
NanoMaterials: SERS-active substrates
Enhancement Distance Dependance
Stiles P.L. et all, Annual Review of Analytical Chemistry, 1, 2008, p.601-26
Surface Enhanced Resonance Raman Spectroscopy (SERRS)
RRS provides additionalEF = 10 2 -10 6
Resonance
Resonance
No resonance
Due to matching of excitation to absorption of a specific part of the molecule, theRaman spectrum associated with this part of the molecule is selectively enhanced
Creation of Metal Nanostructures: AgNP AgNP = silver nanoparticlesSynthesis● Reduction of Ag salt (AgCl, Ag2SO4, AgNO3) or Ag2O● Reducing agents (H2 gas, glucose, NaBH3)
Thermal Evaporation/Deposition
Electrochemical Roughening
Delivering molecules to metal structures
laser serslaser sers
laser sers
laser sers Measure in the solvent
RamanSelection Rules
Wavelength RangeVaries depending on laser (UV-Vis-NIR)
Advantages● Water is not a problem● Special situations very sensitive● Remote is possible● Visible excitation (cheaper, more robust
optics in theory)● Relatively easy to measure bands below
400 cm-1
● Signal enhancement is possible● With SERRS single molecule detection!
DisadvantagesVery low signal, low sensitivityFluorescence interferenceGas is “difficult”
IR Selection Rules
Wavelength Range2500 – 25000 nm
Advantages● More sensitive● Remote operation possible● More robust for quantitative analysis
(absorbance), FT
Disadvantages● Too sensitive to water● Major change to spectrometer to get
below 400 cm-1
ν =
ReferencesMcCreery 2000, DOI:10.1002/0471721646
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