5. The device of claim 2, wherein said solvent comprises one or more compounds selected from the group consisting of water, dimethyl formamide, dimethylsulfoxide, isopropa› nol, acetone, tetrahydrofuran, and combinations thereof. 

6. The device of claim 1, wherein said pH-control sub- stance is triggered with electromagnetic radiation to gener› ate said acid or said base. 

7. The device of claim 6, wherein said pH-control sub› stance is a photoacid generator or a photobase generator. 

8. The device of claim 1, wherein said pH-control sub- stance is triggered with a change in temperature to generate said acid or said base. 

9. The device of claim 1, wherein said trigger source is configured to uniformly trigger said pH-control substance in space within said reaction microfluidic region. 

10. The device of claim 1, wherein said trigger source is configured to trigger said pH-control substance in a spatial pattern within said reaction microfluidic region. 

11. The device of claim 1, wherein said trigger source is disposed in electrical communication with a controller to maintain pH within said reaction microfluidic region within a preselected pH range. 

. 12. Th~ device of claim 1, wherein said trigger source is disposed m electrical communication with a controller to oscillate pH within said reaction microfluidic region. 

13. The device of claim 1, wherein said trigger source is disposed in electrical communication with a controller to maintain pH rate of change within said reaction microfluidic region below a preselected maximum pH rate of change. 

14. The device of claim 1, wherein said reaction micro- fluidic region contains one or more particle arrays. 

15. The device of claim 1, wherein said device further comprises a particle-array outlet port disposed in fluid communication with said reaction microfluidic region. 

16. The device of claim 15, wherein said device is configured for continuous operation. 

17. ~~e device of claim 1, said device further comprising an add1t10nal reaction microfluidic region disposed in fluid communication with said reaction microfluidic region. 

18. A method of using the device of claim 1, said method comprising: 

preparing_ or obtaining a first fluid comprising a plurality of particles to be assembled, a solvent, and said pH› control substance; 

pr~pari~g _or o?tain)ng a second fluid that is substantially 1mm1sc1ble m said first fluid; 

flo~ing s~id first fluid through said first-fluid inlet port, mto said droplet-generating microfluidic region• 

flowing said second fluid through said second-fluid inlet port, into said droplet-generating microfluidic region, th~reby generating a plurality of droplets containing said first fluid, wherein said droplets are phase-sepa› rated from said second fluid; 

flowing said droplets into said reaction microfluidic region; 

triggering generation of an acid or a base from said pH-control substance, within said reaction microfluidic region; and 

titrating pH within said reaction microfluidic region towards the average isoelectric point of said particles. 

19. The method of claim 18, wherein said first and second fluids are flowed simultaneously into said droplet-generating microfluidic region. 

20. The method of claim 18, wherein said particles assemble into a particle array. 

21. The method of claim 20, wherein said particle array is recovered from said reaction microfluidic region. 

22. The method of claim 18, wherein said particles are characterized by an average particle size from about 1 nanometer to about 100 microns. 

23. The method of claim 18, wherein said particles contain a core and a shell, wherein said core comprises a metal, a polymer, a cermnic, or a combination thereof and wherein said shell comprises an oxide, a fluoride, a sulfide, or a combination thereof. 

24. ~he ~ethod of claim 18, wherein said titrating pH :esults 1~ a t1~rated pH within 1.5 pH units of said average 1soelectnc pomt of said particles. 

_2~. The method of claim 24, wherein said titrated pH is within 0.5 pH units of said average isoelectric point of said particles. 

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