Firstly, the device would have to be compatible with brain waves. The desire to want some knowledge starts at the brain, and therefore, that device would have to be connected to the neurons that deal with reasoning. So, once that that part of the brain has been alerted of the desire for knowledge, there would need to be a way to communicate that to the Google device.
Now, as for the way that would be communicated. I think an action potential could be a way to do it. My reasoning is this: an action potential is known as the all-or-none reaction. Binary code, the code used to program computers, is made up of 1s and 0s, a sequence of "yes"s and "no"s. Therefore, a sequence of action potentials could be the biological equivalent of binary code. Once the action potentials are received, the device would use binary code to translate it into a word search (imagine it as a search bar and the action potentials are the keyboard).
You could also develop this further. In fact, that is my challenge to you guys. All of the other logistics (size, the device's information storage facility, privacy, etc.), I challenge you to find solutions to them. For example, for the device's information facility: I would I say that it doesn't need one, if it uses the brain's memory as it's hard-drive.