Dynamic Light Scattering (aka QLS, PCS)

Transcription

1 Dynamic Light Scattering (aka QLS, PCS) Oriented particles create interference patterns, each bright spot being a speckle. The speckle pattern moves as the particle move, creating flickering. All the motions and measurements are described by correlations functions G (!)- intensity correlation function describes particle motion G 1 (!)- electric field correlation function describes measured fluctuations Which are related to connect the measurement and motion ) G ' (! ) B 1 ( * g (! ) % & 1 $ "# Analysis Techniques: Treatment for monomodal distributions: linear and cumulant fits Treatment for non-monomodal distributions: Contin fits It is also possible to measure other motions, such as rotation.

2 Particles behave like slits, the orientation of which generates interference patterns Generates a speckle pattern Various points reflect different scattering angles

3 Movement of the particles cause fluctuations in the pattern The pattern fluctuates Movement is defined by the rate of fluctuation

4 Measure the intensity of one speckle Experimentally, the intensity of one speckle is measured

5 Order of magnitude for time-scale of fluctuations fluctuations occur on the time-scale that particles move about one wavelength of light -x /. Assuming Brownian motion of the particles The time-scale is: + -x, ) Dt. ~ 500 nm Change on the msec time frame + 05, 5x10 cm / m sec t / x10 cm / s D for ~ 00 nm particles

6 How is the time scale of the fluctuations related to the particle movement? Requires several steps: 1. Measure fluctuations an convert into an Intensity Correlation Function. Describe the correlated movement of the particles, as related to particle size into an Electric-Field Correlation Function. 3. Equate the correlation functions, with the Seigert Relationship 4. Analyze data using cumulants or CONTIN fitting routines

7 Math/Theory Application/Optics Data Analysis texts: Light scattering by Small Particles by van de Hulst Dynamic Light Scattering with applications to Chemistry, Biology and Physics by Berne and Pecora

8 First, the Intensity Correlation Function, G (!) Describes the rate of change in scattering intensity by comparing the intensity at time t to the intensity at a later time (t +!), providing a quantitative measurement of the flickering of the light Mathematically, the correlation function is written as an integral over the product of intensities at some time and with some delay time,! I(t) I(t+!) I(t+! ) G 1 T (! ) ) 1 I( t )I d T 0 + t (!,! Which can be visualized as taking the intensity at I(t) times the intensity at I(t+!)- red), followed by the same product at I(t+t )- blue, and so on

9 The Intensity Correlation Function has the form of an exponential decay plot linear in! 1.8e+8 1.6e+8 The correlation function typically exhibits an exponential decay G(!) 1.4e+8 1.e+8 1.0e+8 1.8e+8 plot logarithmic in! 8.0e e+8 Tau (sec) G(!) 1.4e+8 1.e+8 1.0e+8 8.0e Tau (sec)

10 Second, Electric Field Correlation Function, G 1 (!) It is Not Possible to Know How Each Particle Moves from the Flickering Instead, we correlate the motion of the particles relative to each other

11 Integrate the difference in distance between particles, assuming Brownian Motion The electric field correlation function describes correlated particle movement, and is given as: 1 T G 1(! ) ) 1 E( t )E d T 0 + t (!,! Constructive interference G 1 (t) decays as and exponential with a decay constant 345for a system undergoing Brownian motion G 1 (! ) ) exp 03! Destructive interference

12 The decay constant is re-written as a function of the particle size The decay constant is related by Brownian Motion to the diffusivity by: 3 ) 0Dq q ) 46n < = 9 sin : 7. ; 8 with q reflecting the distance the particle travels and the application of Stokes-Einstein equation Boltzmann Constant temperature D ) kt 66r ) thermodynamic hydrodynamic viscosity particle radius

13 Rate of decay depends on the particle size 1.0 G (!) large particles diffuse slower than small particles, and the correlation function decays at a slower rate Tau large particle and the rate of other motions (internal, rotation ) G (!) small particle 0.0 1e-6 1e-5 1e-4 1e-3 1e- 1e-1 Tau

14 Finally, the two correlation function can be equated using the Seigert Relationship Based on the principle of Guassian random processes which the scattering light usually is The Seigert Relationship is expressed as: Intensity I ) E ) E > E * G ' (! ) B 1 ( * g (! ) % & 1 ) $ "# Intensity Correlation Function (recall: this is measured) Electric Field Correlation Function (recall: this is what the particles are doing) where B is the baseline and * is an instrumental response, both of which are constants

15 G (!) intensity correlation function measures change in the scattering intensity G 1 (!) electric field correlation function describes correlated particle movements The Seigert Relationship equates the functions connecting the measurable to the motions G ' (! ) B 1 ( * g (! ) % & 1 ) $ "#

16 Math/Theory Application/Optics Data Analysis

17 So, consider a simple example of the process Measure the intensity fluctuations from a dispersion of particles.

18 Commercial Equipment Need laser, optics, correlator, etc Commercial Sources Brookhaven Instruments Malvern Instruments Wyatt Instruments (multiangle measurements, HPLC detectors) ALV (what we have) Costs range $50K to $100K

19 Light Source Instrumental Considerations Monochromatic, polarized and continuous (laser) Static light scattering goes as 1/! 4, suggests shorter wavelengths give more signal typical Ar + ion laser at 488 nm Dynamic light scattering S/N goes as!, while detector sensitivity goes as 1/!, so wavelength is not too critical. HeNe lasers are cheap and compact, but weaker (! = 633 nm) Power needed depends on sample (but there can be heating!) Calculation of G(") depends on two photons, and so on the power/area in the cell. Typically focus the beam to about 00 #m Sample can be as small as 1 mm in diameter and 1 mm high. Typical volumes 3-5 ml.

20 Instrument Considerations Need to avoid noise in the correlation functions Dust! Usually adds an unwanted (slow) component See in analysis some software help AVOID by proper sample preparation when possible Poisson Noise counting noise, decreases with added counts, important to have enough counts; typically 10 7 over all with 10 6 at baseline Stray light adds an unwanted heterodyne component (exp (-$) instead of exp (-$). Avoid with proper design

21 Need to calculate which is approximated by Correlators G 1 T (! ) ) 1 I( t )I d T 0 + t (!,! N ( l arge) 1 G (! ) I( ti ) I( ti (! ) N so calculate by recording I(t) and sequentially multiplying and adding the result. To do in real time requires about ns calculations thus specialized hardware i) 1 Pike 1970s (Royal Signals and Radar Establishment, Malvern, England) Langley and Ford (UMASS)? Brookhaven 1980 s Klaus Schatzel, Kiel University? ALV

22 Autocorrelation function is collected The auto-correlator collects and integrates the intensity at the different delay times,!, all in real time! (sec) G +!, Each point is a different!. G(!) 1.8e+8 1.6e+8 1.4e+8 1.e+8 1.0e+8 8.0e Tau (sec) e e E E E E E E E E E E E E E E E E E E E E E E E E+007

23 then, create the raw correlation function Evaluate the autocorrelation function from the intensity data 1.8e+8 1.6e+8 G(!) 1.4e+8 1.e+8 1.0e+8 8.0e Tau (sec)

24 then, normalize the raw correlation function through some simple re-arrangements C G (! ) 0 B B 0 3! +!,) ) * e c(t) * * is usually less than unity, from measuring more than one speckle B B should go to zero ! (sec)

25 General principle: the measured decay is the intensity-weighted sum of the decay of the individual particles nm 00 nm 300 nm 400 nm c(t) nm tau (sec) Recall that different size particles exhibit different decay rates.

26 Expressed in mathematical terms g 1 (!) can be described as the movements from individual particles; where G(3) is the intensity-weighted coefficient associated with the amount of each particle. g 1 +!, G + 3, i i e 03i! For example, consider a mixture of particles: 0.30 intensity-weighted of 100 nm particles, 0.5 intensity-weighted of 00 nm particles, 0.0 intensity-weighted of 300 nm particles, 0.15 intensity-weighted of 400 nm particles, 0.10 intensity-weighted of 500 nm particles.

27 A sample correlation function would look something like this Short times emphasize the intensity weightedaverage wieghted sum of the individual decay Recall sizes 0.30 (100 nm) 0.5 (00 nm) 0.0 (300 nm) 0.15 (400 nm) 0.10 (500 nm) c(t) nm 00 nm 300 nm 400 nm 500 nm tau (sec) long times reflected the larger particles

28 Math/Theory Application/Optics Data Analysis

29 Finally, calculate the size from the decay constant 3 =??? in sec -1 (experimentally determined) D =??? in cm sec -1 D ) 3 q Diffusivity is determined need refractive index, wavelength and angle q ) 46n < = sin :. ; r =??? in cm r kt ) 66D Calculate the radius, but need the Boltzmann constant, temperature and viscosity

30 What is left? Need a systematic way to determine 3 s the distribution of particle sizes defines the approach to fitting the decay constant Monomodal Distribution Linear Fit Cumulant Expansion Non-Monomodal Distribution Exponential Sampling CONTIN regularization

31 What is left? Need a systematic way to determine 3 s First, consider the monomodal distribution, where the particles have an average mean with a distribution about the mean (red box, first) Monomodal Distribution Linear Fit Cumulant Expansion Non-Monomodal Distribution Exponential Sampling CONTIN regularization

32 Simplest- the basic linear fit Assumes that all the particles fall about a relatively tight mean Take the logarithm of the normalized correlation function G (! ) B ln < 0 9 : 7 ) ln * 0 q ; B 8 D! C(!) ln * ln C(!) e-1 Slope = -Dq C(!) but, need Long! for a good B C(!) 1e- 1e e Tau Long! there is just noise Tau

33 Cumulant expansion Assumes that the particles distribution is centered on a mean, with a Gaussian-like distribution about the mean. Where to start g +!, G+ 3, ) A 03! 1 d 0 1 e 3 Integral sum of decay curves Larger particles are seen more Probability Density Function (Coefficients of Expansion) + 3, ) M P( q )S( q ) G + 3, 6 N( R) R : solid + 3, 4 N( R) R : hollow shell (vesicle) G / G / Intraparticle Form Factor And Interparticle Form Factor that both DEPEND ON q

34 Then, re-arrange the Seigert Relationship in terms of a cumulant expansion Recall that the correlation function can be expressed as +!, G +!, B lnc ln ' 0 ) $ ) ln * 0 ln g 1 %& B "# +!, Cumulant expansion is a rigorous defined tool of re-writing a sum of exponential decay functions as a power series expansion so, that the sum from the previous page is replaced by the expansion (GET BACK HERE IN A FEW MINUTES) g +, ) A i 3!! e P+ 3, d 0 +, A n + i!, A + i!, ln g1! kn ) 1 kn d! n! n! n) 0 0 n

35 need to carry through some mathematics First, define a mean value g 1 03! 0 +, + 303,!! ) e e 3 is the mean gamma Note: power series expansion 0 x x e x / 10 x ( 0 (...! 3! 3 Second, substitute the power series for the difference term (second term) g 1 A 03! 03! +!, ) G+ 3, e d G+ 3, e , 1 3 ) , ' $ 1 % 1! (! 0..." 3 0 &%! d "# A

36 g 1 A Cumulant Expansion (more) 03! 03! +!, ) G+ 3, e d e G+ 3, , 1 3 ) 0 Working through the integrals , ' $ 1 % 1! (! 0..." 3 0 %&! d "# A g 1 +!, ) e 03! < : k k 10 0 (! 0 3! ;! 3! ( Such that k is the second moment, k 3 is the third moment, k +, , 3 ) A 1G 3 d 0 k 3 3 +, , 3 ) A 1G 3 d 0

37 Cumulant Expansion (even more) +, 1 $ ' ) ( % %& ' G 0 B 0 ln 1 ln * ln e "# 3 B &%!! < : 1( ; k!! 0 3 k3 3!! 3 ( 9$... 7 " 8" # a b x Note: ln of products Note: power series expansion ln( ab ) ) ln a ( lnb 1 ln( 1( x ) / x 0 x (... Note that x terms >> x terms, so that x are negligible

38 Cumulant Expansion (more ) +!, G B ln ' $ ) ln 0 3 ( K 0... %& 0 * B "#!! Note: Multiplied by intercept average decay polydispersity Polydispersity index C ) k 3 and indicates the width of the distribution C ) is mono-dispersed

39 390 nm Beads Sample of Cumulant Expansion linear +!, ' G 0 B ln % & B $ " # ) ln * 0 3! Gamma quadratic cubic quartic +!, B$ ' G 0 ln ln K % ) * 0 3! ( B "! & # 3 ' G 0 ln ln K 3 % ) * 0 3! (! 0! & B " # 3 ' G ln 0 % & B +!, B$ K 3 3 3! 3 ~ Kurtosis 4 ( 4! 1 +!, B$ K 3 K 4 " # ) ln * 0 3! ( K! 0 ~ Poly ~ Skew

40 Examine residuals to the fit uncorrelated correlated

41 What is left? Need a systematic way to determine 3 s Second, consider the different non-monomodal distribution, where the particles have a distribution no longer centered about the mean (red box, next) Monomodal Distribution Linear Fit Cumulant Expansion Non-Monomodal Distribution Exponential Sampling CONTIN regularization Multiple modes because of polydispersity, internal modes, interactions all of what make the sample interesting!

42 Exponential Sampling for Bimodal Distribution g 1 03! +!, a e i i Assume a finite number of particles, each with their own decay e.g. bimodal distribution To be reliable the sizes must be ~5X different

43 Pitfalls Correlation functions need to be measured properly a) Good measurements with appropriate delay times b) Incomplete, missing the early (fast) decays c) Incomplete, missing the long time (slow) decays

44 CONTIN Fit for Random Distribution Laplace Transform of f(t) F A 0 st + s, ) LDf + t, E ) e f +,dt t In light scattering regime. 1 0 Note: Fourier Transform A G+ F, ) G f+ t, D E ) e 0iFt 1 f+ t,dt 0A size distribution function g 1 A 3 +, ) D + 3, E ) 0!! L G e G+, d So, to find the distribution function, apply the inverse transformation which is done by numerical methods, with a combination of minimization of variance and regularization (smoothing) , ) L D g1 +!, E G 0

45 CONTIN Developed by Steve Provencher in 1980 s Recognize that g 1 A 3 +, ) D + 3, E ) 0!! L G e G+, d is an example of a Fredholm Integral where F ( r) ) K( r, s) A( s) ds 1 measured object of desire defines experiment This is a classic ill-posed problem which means that in the presence of noise many DIFFERENT sets of A(s) exist that satisfy the equation

46 So how to proceed? CONTIN (cont.) 1. Limit information i.e., be satisfied with the mean value (like in the cumulant analysis). Use a priori information Non-negative G($) (negative values are not physical) Assume a form for G($) (like exponential sampling) Assume a shape 3. Parsimony or regularization Take the smoothest or simplest solution Regularization (CONTIN) ERROR = (error of fit) +function of smoothness (usually minimization of second derivative) Maximum entropy methods (+ p log (p) terms)

47 Analysis of Decay Times First question: How do decay times vary with q? Diffusion (translation) Slope = D app Finite Rotational Diffusion Slope = D app 3 3 D r g 1 0 +, + Dq 6,!! / e 0 D r q q $= D app q where D app is a collective diffusion coefficient that depends on interactions and concentration Rotational diffusion can change the offset of the decay can also observe with depolarized light

48 Not spheres but still dilute, so D = kt/f Cylinders 1 D ) ( 3 + D D, 1 D 1 Worms D ) ln + L, D KT 36H o L D 1 Prolate D ) kt f b a Shape factor: A hydrodynamic term that depends on shape f ) + b, a /3 + b, 1/ $ %& ' 0 1 a "# < : 1 ( ' 1 0 : %& a ln : b : a ; + b, $ "# 1/

49 Concentration Dependence In more concentrated dispersions (and can only find the definition of concentrated generally by experiment ), measure a proper D app, but because of interactions D app (c) Again, D = <thermo>/<fluid> = kt(1 + f(b) + )/f o (1 + k f c + ) So D app = D 0 (1 + k D c + ) with k D = B k f % like a second virial coefficient for diffusion partial molar volume of solute (polymer or micellar colloid)

50 Driving force = Virial Coefficient < : ; J 9 7 IK 8 I 0 1 T at low density ) kt[1 ( 46K drr 1 ( g( r) 01] so for low density < IJ 9 : 7 / kt[1 ( KB ; IK 8 T / kt[1-46k (...] 1 (g(r) -1)r dr (...] where B ) ( g( r) 01) r dr

51 Multiple Scattering single scattering multiple scattering Three approaches Experimentally thin the sample or reduce contrast Correct for the effects experimentally Exploit it!

52 Diffusing Wave Spectroscopy (DWS) In an intensely scattering solution, the light is scattered so many times the progress of the light is essentially a random walk or diffusive process Measure in transmission or backscattering mode Probes faster times than QLS See Pine et al. J. Phys. France 51 (1990)

53 Summary Oriented particles create interference patterns, each bright spot being a speckle. The speckle pattern moves as the particle move, creating flickering. All the motions and measurements are described by correlations functions G (!)- intensity correlation function describes particle motion G 1 (!)- electric field correlation function describes measured fluctuations Which are related to connect the measurement and motion G ' (! ) B 1 ( * g (! ) %& 1 ) Analysis Techniques: Treatment for monomodal distributions: linear and cumulant fits Treatment for non-monomodal distributions: Contin fits Interactions, polydispersity, require careful modeling to interpret $ "# Other motions, such as rotation, can be measured