Class 5 Immuno-assay with magnetic nanoparticle tags

Gaster et al Nature Medicine 15:1327 (2009)

Basic ideaGiant magneto resistance (GMR)Sensor characterization

Sandwich immunoassay with superparamagnetic particle tags and GMR detection of magnetic labels

Reaction well: tygon tubing glued to GMR sensor surface

5mm

3mm

well vol ~125ml

sample vol 50ml

sample height ~1.5mm

What is GMR*? electrons interact differently with mag.materials depending on alignment of their spin with local magnetic field – more scattering (moreresistance) when spin and field are parallel

*used in hard drive read heads, Nobel prize in physics 2007

GMR “spin valve” sensor

17nm

40nm

Apply H = Hysinwt j then R oscillates with freq. w (or 2w)

+ -

50nm Applied HaFe

Superparamagnetic: mag moments not aligned (at RT) in absence of ext. field

Mag field from beads (if present) adds to GMR resistance

How far from bead is its magnetic field felt?

dipole field ~(a/r)3, so ~100nm for ~50nm beads

How wide is sensor strip?

How should bead effect on resistance vary with # beads?

Previous expt used 16nm beads on 200nm-wide sensors

Change in resistance was ~number of beads stuck

Signal processing

They apply current I, measure V (=IR) as surrogate for R

They apply oscillating H(w1) -> oscillating R(w1), V(w1)

They also vary I(w2) -> V varying with w1, w2, w1+w2

Measure rms amplitude of DV(w1+w2)

Repeat after adding allowing beads to attachi.e. measure (D DV) due to beads

Other things to worry about

Temperature effects T may change due to environmental drift or

joule heating with current thru sensor T alters bulk resistance DVBR =aDT T also alters GMR effect DVGMR(w) =bDT They know a, use DV(w=0) to est. local DT, then

correct DV(w1+w2) for temp effect on GMR Caveat: the magnitude of temperature effects are

comparable to changes in V from beads

How do GMR sensor results differ from ELISA?Does GMR extension to right => not all receptors bind

analyte at 500pM? If all bound at 5nM, whatfraction should be bound at 5fM?

How may receptors are bound at 5fM? Why is DDV ~ [c0]1/3?

Aside on fraction of receptors that bind analyteand fraction of analyte that binds receptors

ar/r0 = a0/(KD+a0+r0) [1+ a0 r0 /(KD+a0+r0+)2 + … ]

ar/a0 = r0/(KD+a0+r0) [1+ a0 r0 /(KD+a0+r0+)2 + … ]

a0 = initial conc of analyte (per sample vol)

r0 = initial conc of receptors (per sample vol)

Useful when correction term in power series is smalland sample volume is fixed (i.e. no flow)

See spread sheet!

Why is limit of detection for CEA 25X lower than for lactoferrin?

Time course of DDV Why so fast?Suppose conc. of beads is 4nM, KD of biotin-SAbond is 10-15M, and kon is the usual 106/Ms In this case c0/KD >>1 so trxn = (1/koff)/(1+c0/KD) simplifies to ~1/(c0kon)

Amplification = after beads bind, add any protein w/several biotins, then more SA-magnetic nanoparticlesCan they detect 50aM analyte? If so, how many analyte

molecules should be bound?

Shows greatreproducibilityin different conditions, butare results forCEA the same as in Figure 2?

GMR immunoassay for serum CEA assay can be used to monitor growth of tumor that secretes CEA in mice

How big a tumor would this correspond to in humans, assuming mice and men are proportional in all relevantways, the 21 day mouse tumor is 1 g, and mice weigh 30g versus 60kg for men? Might we need more sensitivity?

Conclusions

1. GMR immuno-assay has much larger dynamic range than ELISA; this could be very important inmultiplex assays for proteins with vastlydifferent concentrations in same sample

2. GMR assay is sensitive, but maybe less than claimed3. How costly are sensor chips?4. How costly is sensor?5. How reliable is data processing, e.g. for temp.

correction?

Next week: immuno-assay with single-molecule sensitivity based on fluorescence labels and Total Internal Reflection Fluorescence Microscopy (TIRFM)

Read Jain et al Nature 473:484 (2011)

Basic idea – capture analyte on transparent surface introduce fluorescent label (e.g. on second ab) record fluorescent image in microscope

sample negative control

TIRF microscopy reduces background, allowingdetection of single fluorescent molecules

Jargon protein names: YFP, PKA, ADAP, mTor, etc. epitopes (small chemical features, can be peptides,

that antibodies bind to): FLAG, HA fluorescent proteins (e.g. from jellyfish, corals): often

named for emission color yellow (YFP), red (mCherry) IP = immunoprecipitation, here usually means

capture of analyte on surface by antibody FRET – Fluorescence Resonance Energy Transfer:

when different fluors are within nm of eachother, excited state can transfer -> altered em. color

photobleaching – light-induced chem. change killing fluor.

Authors describe technique mainly for researchpurposes: e.g. to detect what other proteinsa test protein binds to, or how many moleculesin a complex

Our focus: how does this method compare to othersas a sensor

Issues to think about as you read: background, dynamic range, field of view,potential for automation, cost