Probably any solvent other than water will kill the bacteria.

One possible solution, if the bacteria is well surrounded by your NP, could be to add water with some kind of mild surfactant (lipids similar to ones employed for forming liposomes). This could put your NP-bacteria "aggregation" in a sort of "micelle" that may help for your studies.

This is only an idea that came to my mind, I don't know if something similar has done before or if it will work at all for you.

Good luck!

     Without details concerning the chemistry of your system, it's difficult to make specific suggestions.  However, you might try something that is biocompatible and water soluble like polyethylene glycol (PEG).  Perhaps a PEG block copolymer with a short hydrophobic block as the second component (or a PEG functionalized with a hydrophobic alkyl chain at one of its ends).  For example, Sigma-Aldrich sells a variety of PEG derivatives with Mn values of a few hundred, like poly(ethylene glycol) monolaurate or poly(ethylene glycol) monooleate.  If the hydrophobic laurate or oleate segments can intercalate/partition into the hydrophobic coating of the nanoparticles, the result would essentially be physisorptive attachment of the water soluble PEG portion as a coating over the hydrophobic nanoparticles.  PEG, laurate, and oleate are all biocompatible materials. This should improve water dispersion ability of the system. 

     An alternative would be to modify the chemistry of your hydrophobic coating.  For example PEG is also available with a variety of reactive end groups, like thiol, amine, or carboxylic acid.  If the terminal functional group of your hydrophobic coating were a disulfide, you could convert it to a thiol with a mild reducing agent and then react with the PEG-thiol to form a new disulfide covalently attaching the PEG to the periphery of your hydrophobic coating.  PEG is also available with acetylene or azide endgroups (for biocompatible click chemistry) or maleimide (another thiol reactive reagent).  I suggest you check out bioorthogonal bioconjugation reagents if you want to pursue this covalent route to see what is available and most compatible with your specific system (An alternative would be to look at some photochemistry to convert a hydrophobic functional group to a hydrophilic or reactive one after separation of the bacteria-nanoparticle complex for dilution in water.)

    In summary, the physisorptive route is probably the easiest and cheapest to try, but covalent binding would be more permanent.  The choice all depends on the chemistry of your specific system and intended application conditions.

Dear All  Mr.an Philippe ,Mr. Dressick and Mr. Diaz-Moscoso

Thanks for your information 

You are all suggestion  valuable but still it seems to be some problems

In my experiment my final complex is bacteria-fluorescent hydrophobic nano-particles. Here bacterial hydrophobic components adhesion with hydrophobic NPs through hydrophobic interaction. 

--According you all suggestion 

 if i use surfacants,PEG,PROTEINS ,it may effect on interaction between bacteria-fluorescent hydrophobic nano-particles.There are some literature saying surfactant degradate  the bacteria. Moreover PEG and proteins may shows fluorescence it may influence on my complex fluorescence spectra. 

I don't think the PEG materials I suggested have any fluorescence.  There is really no strong chromophore in the PEG itself to absorb most UV-visible light used for emission spectroscopy of biomolecules, so it is difficult to see how it would fluoresce.  If you are concerned about the double bond in the oleate group, use the laurate instead since it is a saturated hydrocarbon and will not absorb UV-visible light.  if you are still concerned then it seems you will have to explore the photochemistry/protecting group option I suggested instead.  Specifically, select a hydrophobic coating for the nanoparticle that has a hydrophilic terminal group.  Protect that terminal group with a hydrophobic material and use it in your process.  When the bacteria-nanoparticle complex is isolated, treat with a reagent (or light) to remove the protecting group and regenerate the hydrophilic terminal group to provide the water solubility for dilution.  One system that comes to mind in acetals.  Specifically, if you use a material to coat the nanoparticles that has a terminal alcohol group, you can protect the alcohol by reaction with a ketone like acetone to make an acetal.  This could be done before the coating is applied to the nanoparticles used in your process.  Acetals are stable in neutral and alkaline pH solutions normally encountered for biomolecules.  However, in acidic solutions they hydrolyze to release the ketone and alcohols.  I would suggest a small volatile molecule like acetone as a ketone candidate.  In the small amounts released from the monolayers on your nanoparticles it may not hurt the biomolecules.  Other potential systems are carbonates, which can decompose to alcohols and carbon dioxide with the appropriate trigger.  I would suggest you look at the topic of self-immolative polymers for biocompatible systems and trigger ideas, as well as various organic protecting groups and deprotection strategies.  Either may provide a chemistry compatible with your system that could do the job.  Once again, without details of your system it is hard to make specific suggestions.  Good luck.

I would start with the question - are your bacteria still viable after complexing with fluorescent dye?

Thank you Dear Walter J Dressick for your valuable suggestion .

Howeever in my study their is no acetate and alcohol chemistry study .

Octadecyl amine i used to modify the  nano material.

Dear Titus Sobisch

I don't use any fluorescent dye.

My nano-material is it self fluorescent and it is hydrophobic material.

After bacteria complex with nanomaterial still bacteria alive. 

Perhaps I was not sufficiently clear.  My suggestion of the acetal chemistry was a SPECIFIC example of a more GENERAL suggestion.  That is, if you want to change solubility of your nanoparticle-bacteria aggregates, you will need to change the chemistry of the nanoparticle coating.  I suggested that you examine some classical functional group protection-deprotection schemes and photochemistry that might allow you to do that, citing acetal chemistry as one simple example.  Doings so may require you to replace the oleylamine you use with a different hydrophobic coating that would permit a change in wettability from hydrophobic to hydrophilic via treatment with a mild reagent or via photolysis.  Oleylamine (i.e., (Z)-Octadec-9-enylamine) contains a double bond near the middle of its alkyl chain.  If you must use oleylamine or don't want to consider an exchange for another coating species like an acetal that could be readily converted to a hydrophilic species, you need to modify that double bond to increase hydrophilicity.  For example, you could oxidize it to a water soluble carboxylate or hydroxylate it to a vicinal-diol, provided you can find a mild reagent that will not harm the bacteria.  I don't know what that reagent is.  That is why I suggest examining the organic functional group protection-deprotection literature and photochemistry literature.  There is also a lot of effort to covalently attach biomolecules to surfaces, etc..., so reagents have been developed to do that without denaturing or harming the biomolecules.  A good place to start looking for these might be a journal like Bioconjugate Chemistry.

Good advise, except there should be a correction - not change in solubility but change in dispersibility is the goal. Nonetheless, you refrained from using surfactants, I would assume adsorption of relatively low concentration of nonionic surfactant like from the Tween type or alkyl polyglycoside will not change much the viability of your bacteria but make your complex hydrophilic in aqueous media.

Thanks for the correction, Dr. Sobisch.  I should have known better than to confuse solubility and dispersibility, since I work a lot with colloids.