Here is a paper on pH-dependence of  zeta potential for various natural cellulose-based fibres. Take a look.

You may take a look at an article in the Journal of the Chilean Chemistry:

A MINIREVIEW OF CELLULOSE NANOCRYSTALS AND ITS POTENTIAL INTEGRATION AS CO-PRODUCT IN BIOETHANOL PRODUCTION

if it is an original cellulose without modification, the zeta potential would be low around 0 V because it is supposed to be neutral.

If you need a really precise answer it's a good idea to determine it yourself from the samples of interest. The answer really depends on the chemical treatments that the sample has gone through but generally most cellulosic fibers tend to have a negative charge in water. This paper by Junka et al. may prove quite useful. http://link.springer.com/article/10.1007%2Fs10570-013-0043-z#page-1

I agree on the two last.

We have measured the zeta potential of cellulose coated magnetic particles and it was negative (-22.9 mV).

In solution, zeta potential won't tell you the overall surface charge. It will give you an estimate of the electrical *potential* *near* the surface. You will need to make measurements across a wide range of pH and find the right mathematical model to convert your raw measurement data (usually electrophoretic mobility via light scattering) to charge. No current commercial instrument does this correctly.

Use acid/base titration.

In the solid state, the mechanisms for the formation of surface charge are not the same as in solution. Solid surface charge will be difficult to control (ambient humidity, touching/rubbing the surface etc).

Of course, this all depends on why you need to know...

The theoretical background of the zeta potential is common for colloids and solids and based on the model of the electrochemical double layer (EDL) at the solid-liquid interface. Fig. 1 shows a schematic drawing of the interfacial charge distribution at a solid-water interface. The surface charge, generated, e.g., by the interaction of water with surface functional groups, gets partially compensated in a stagnant layer of water and counter-ions. Full compensation of the surface charge requires additional counter-ions that are accumulated in the diffuse layer. The distribution of counter-ions (cations in Fig. 1) and co-ions (anions in Fig. 1) in this diffuse layer differs from the ion distribution in the bulk aqueous phase. Upon a relative motion between the solid and the liquid phase, an electrokinetic effect is established. The measuring parameters of electrophoretic mobility or streaming potential mentioned above belong to the series of such electrokinetic effects.

What about lignocellulos9c nano particles (non bleached nano particles ?)