Trustees may invest trust funds in bonds and other securities issued by this state bearing a lower rate of interest than 7 percent per annum and shall, in the settlement of their accounts on the funds so invested, be chargeable with no greater interest than that received from the state; provided, however, that the trustee shall, within 30 days after the investment, make a return to the judge of the probate court of the amount and character of the bonds purchased and the price paid.